toolchain-bootstrap: force target triple when building on Apple

This used to "just work." But for reasons unknown to me, GitHub Actions
Apple ARM runners result in `config.guess` thinking it is a x86-64
machine and this causes LLVM's CMake to build x86-64 binaries which
target x86-64 by default!

Our Python code has working target sniffing. So we use that to feed
an appropriate triple into LLVM's CMake.

toolchain-bootstrap/scripts/clang-macos.py

@@ -242,6 +242,11 @@ def build_llvm(
         env["NUM_JOBS_AGGRESSIVE"] = "%d" % max(cpu_count + 2, cpu_count * 2)
         env["MACOSX_DEPLOYMENT_TARGET"] = "11.0"
 
+        if ARCH == "aarch64":
+            env["HOST_TRIPLE"] = "arm64-apple-darwin23.2.0"
+        else:
+            env["HOST_TRIPLE"] = "x86_64-apple-darwin23.2.0"
+
         subprocess.run(
             [str(temp_dir / script)],
             cwd=temp_dir,

toolchain-bootstrap/scripts/clang-macos.sh

@@ -45,10 +45,12 @@ cmake \
     -DCMAKE_CXX_COMPILER=/usr/bin/clang++ \
     -DCMAKE_ASM_COMPILER=/usr/bin/clang \
     -DLLVM_APPEND_VC_REV=OFF \
+    -DLLVM_DEFAULT_TARGET_TRIPLE="${HOST_TRIPLE}" \
     -DLLVM_ENABLE_PROJECTS="bolt;clang;lld" \
     -DLLVM_ENABLE_RUNTIMES="compiler-rt;libcxx;libcxxabi" \
     -DLLVM_ENABLE_LIBCXX=ON \
     -DLLVM_ENABLE_ZSTD=OFF \
+    -DLLVM_HOST_TRIPLE="${HOST_TRIPLE}" \
     -DLLVM_OPTIMIZED_TABLEGEN=ON \
     -DLLVM_LINK_LLVM_DYLIB=ON \
     -DLLVM_TARGETS_TO_BUILD="AArch64;X86" \