Provide prebuilt std for gnullvm targets

src/ci/docker/host-x86_64/dist-various-1/Dockerfile

@@ -55,6 +55,9 @@ RUN ./install-riscv64-none-elf.sh
 COPY host-x86_64/dist-various-1/install-riscv32-none-elf.sh /build
 RUN ./install-riscv32-none-elf.sh
 
+COPY host-x86_64/dist-various-1/install-llvm-mingw.sh /build
+RUN ./install-llvm-mingw.sh
+
 # Suppress some warnings in the openwrt toolchains we downloaded
 ENV STAGING_DIR=/tmp
 
@@ -110,6 +113,9 @@ ENV TARGETS=$TARGETS,armv7r-none-eabi
 ENV TARGETS=$TARGETS,armv7r-none-eabihf
 ENV TARGETS=$TARGETS,thumbv7neon-unknown-linux-gnueabihf
 ENV TARGETS=$TARGETS,armv7a-none-eabi
+ENV TARGETS=$TARGETS,aarch64-pc-windows-gnullvm
+ENV TARGETS=$TARGETS,i686-pc-windows-gnullvm
+ENV TARGETS=$TARGETS,x86_64-pc-windows-gnullvm
 
 ENV CFLAGS_armv5te_unknown_linux_musleabi="-march=armv5te -marm -mfloat-abi=soft" \
     CFLAGS_arm_unknown_linux_musleabi="-march=armv6 -marm" \
@@ -139,7 +145,10 @@ ENV CFLAGS_armv5te_unknown_linux_musleabi="-march=armv5te -marm -mfloat-abi=soft
     CC_riscv64imac_unknown_none_elf=riscv64-unknown-elf-gcc \
     CFLAGS_riscv64imac_unknown_none_elf=-march=rv64imac -mabi=lp64 \
     CC_riscv64gc_unknown_none_elf=riscv64-unknown-elf-gcc \
-    CFLAGS_riscv64gc_unknown_none_elf=-march=rv64gc -mabi=lp64
+    CFLAGS_riscv64gc_unknown_none_elf=-march=rv64gc -mabi=lp64 \
+    CC_aarch64_pc_windows_gnullvm=aarch64-w64-mingw32-clang \
+    CC_i686_pc_windows_gnullvm=i686-w64-mingw32-clang \
+    CC_x86_64_pc_windows_gnullvm=x86_64-w64-mingw32-clang
 
 ENV RUST_CONFIGURE_ARGS \
       --musl-root-armv5te=/musl-armv5te \

src/ci/docker/host-x86_64/dist-various-1/install-llvm-mingw.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+set -ex
+
+archive=llvm-mingw-20231128-ucrt-ubuntu-20.04-x86_64.tar.xz
+curl -L https://github.com/mstorsjo/llvm-mingw/releases/download/20231128/${archive} | \
+tar --extract --lzma --strip 1 --directory /usr/local

src/doc/rustc/src/platform-support/pc-windows-gnullvm.md

@@ -1,6 +1,6 @@
 # \*-pc-windows-gnullvm
 
-**Tier: 3**
+**Tier: 2 (without host tools)**
 
 Windows targets similar to `*-pc-windows-gnu` but using UCRT as the runtime and various LLVM tools/libraries instead of GCC/Binutils.
 
@@ -12,38 +12,52 @@ Target triples available so far:
 ## Target maintainers
 
 - [@mati865](https://github.com/mati865)
+- [@thomcc](https://github.com/thomcc)
 
 ## Requirements
 
-The easiest way to obtain these targets is cross-compilation but native build from `x86_64-pc-windows-gnu` is possible with few hacks which I don't recommend.
+The easiest way to obtain these targets is cross-compilation, but native build from `x86_64-pc-windows-gnu` is possible with few hacks which I don't recommend.
 Std support is expected to be on pair with `*-pc-windows-gnu`.
 
 Binaries for this target should be at least on pair with `*-pc-windows-gnu` in terms of requirements and functionality.
 
 Those targets follow Windows calling convention for `extern "C"`.
 
-Like with any other Windows target created binaries are in PE format.
+Like with any other Windows target, created binaries are in PE format.
 
 ## Building the target
 
-For cross-compilation I recommend using [llvm-mingw](https://github.com/mstorsjo/llvm-mingw) toolchain, one change that seems necessary beside configuring cross compilers is disabling experimental `m86k` target. Otherwise LLVM build fails with `multiple definition ...` errors.
-Native bootstrapping builds require rather fragile hacks until host artifacts are available so I won't describe them here.
+These targets can be easily cross-compiled
+using [llvm-mingw](https://github.com/mstorsjo/llvm-mingw) toolchain or [MSYS2 CLANG*](https://www.msys2.org/docs/environments/) environments.
+Just fill `[target.*]` sections for both build and resulting compiler and set installation prefix in `config.toml`.
+Then run `./x.py install`.
+In my case I had ran `./x.py install --host x86_64-pc-windows-gnullvm --target x86_64-pc-windows-gnullvm` inside MSYS2 MINGW64 shell
+so `x86_64-pc-windows-gnu` was my build toolchain.
+
+Native bootstrapping is doable in two ways:
+- cross-compile gnullvm host toolchain and use it as build toolchain for the next build,
+- copy libunwind libraries and rename them to mimic libgcc like here: https://github.com/msys2/MINGW-packages/blob/68e640756df2df6df6afa60f025e3f936e7b977c/mingw-w64-rust/PKGBUILD#L108-L109, stage0 compiler will be mostly broken but good enough to build the next stage.
+
+The second option might stop working anytime, so it's not recommended.
 
 ## Building Rust programs
 
-Rust does not yet ship pre-compiled artifacts for this target. To compile for
-this target, you will either need to build Rust with the target enabled (see
-"Building the target" above), or build your own copy of `core` by using
-`build-std` or similar.
+Rust does ship a pre-compiled std library for those targets.
+That means one can easily cross-compile for those targets from other hosts if C proper toolchain is installed.
+
+Alternatively full toolchain can be built as described in the previous section.
 
 ## Testing
 
 Created binaries work fine on Windows or Wine using native hardware. Testing AArch64 on x86_64 is problematic though and requires spending some time with QEMU.
-Once these targets bootstrap themselves on native hardware they should pass Rust testsuite.
+Most of x86_64 testsuite does pass when cross-compiling,
+with exception for `rustdoc` and `ui-fulldeps` that fail with and error regarding a missing library,
+they do pass in native builds though.
+The only failing test is std's `process::tests::test_proc_thread_attributes` for unknown reason.
 
 ## Cross-compilation toolchains and C code
 
-Compatible C code can be built with Clang's `aarch64-pc-windows-gnu`, `i686-pc-windows-gnullvm` and `x86_64-pc-windows-gnu` targets as long as LLVM based C toolchains are used.
+Compatible C code can be built with Clang's `aarch64-pc-windows-gnu`, `i686-pc-windows-gnullvm` and `x86_64-pc-windows-gnu` targets as long as LLVM-based C toolchains are used.
 Those include:
 - [llvm-mingw](https://github.com/mstorsjo/llvm-mingw)
 - [MSYS2 with CLANG* environment](https://www.msys2.org/docs/environments)

tests/ui/rfcs/rfc-2627-raw-dylib/dlltool-failed.rs

@@ -3,6 +3,7 @@
 //@ only-gnu
 //@ only-windows
 //@ needs-dlltool
+//@ ignore-llvm
 //@ compile-flags: --crate-type lib --emit link
 //@ normalize-stderr-test: "[^ ']*/dlltool.exe" -> "$$DLLTOOL"
 //@ normalize-stderr-test: "[^ ]*/foo.def" -> "$$DEF_FILE"

tests/ui/rfcs/rfc-2627-raw-dylib/invalid-dlltool.rs

@@ -2,6 +2,7 @@
 
 //@ only-gnu
 //@ only-windows
+//@ ignore-llvm
 //@ compile-flags: --crate-type lib --emit link -Cdlltool=does_not_exit.exe
 #[link(name = "foo", kind = "raw-dylib")]
 extern "C" {