mingw-w64-cross-clang: prefixed cross-compiling wrappers for clang

[jeremyd2019]

Related to discussion #8736.

Based on wrappers from llvm-mingw. Also adds a package that provides "foreign" compiler-rt libs.

The clang wrapper could be simplified (have some of the options removed) if the package is only provided for CLANG*. I haven't tested them under MINGW*.

[jeremyd2019]

This seems to be working quite well. I can use this to cross-compile python. The packaging is a bit odd, but it seems to do the job.

[driver1998]

Wonder how will this work with things like cmake, meson and ninja? Can I use mingw64 cmake with aarch64-w64-mingw32-clang to cross compile ARM64 binaries on x86_64?

[jeremyd2019]

Wonder how will this work with things like cmake, meson and ninja? Can I use mingw64 cmake with aarch64-w64-mingw32-clang to cross compile ARM64 binaries on x86_64?

Note I haven't bothered tested the GCC-based MSYSTEMs with this, they should work but who knows...

But with clang64, absolutely! All the major build systems have a mechanism to configure them to cross-compile. In fact, I had good luck with cmake not even bothering configuring a toolchain file, and instead just specifying options:

        -DCMAKE_{ASM,C,CXX}_COMPILER_TARGET="aarch64-w64-mingw32" \
        -DCMAKE_{ASM,C,CXX}_COMPILER_WORKS=ON \
        -DCMAKE_CROSSCOMPILING=ON \
        -DCMAKE_SYSROOT="/clangarm64" \
        -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER \
        -DCMAKE_FIND_ROOT_PATH_MODE_{LIBRARY,INCLUDE,PACKAGE}=ONLY \

You may not even need to override "compiler works" for normal packages, I was trying to cross-compile libc++ at the time for a prefix lacking it, so the C++ compiler didn't entirely work yet.

[jeremyd2019]

I set up a github repository with a release to keep binaries of this, and set up a pacman repo for the latest version.

The files are in the release https://github.com/msys2-arm/cross-clang/releases/repo. To use, add the following in /etc/pacman.conf:

[cross-clang]
Server = https://github.com/msys2-arm/cross-clang/releases/download/repo
SigLevel = Never