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]

The way I'm doing a "standalone" compiler-rt build here is apparently deprecated. I'll have to investigate how they intend it to be done now, hopefully it doesn't require downloading the llvm sources.

[mati865]

How about installing all architectures supported by us on every architecture instead of creating new compiler-rt packages? It's 35 MiB right now so that would add 70 MiB which doesn't seem so bad considering size and price of today's storage.

[jeremyd2019]

How difficult would that be? Is that possible from the existing mingw-w64-headers-git install or would we need to install the 'foreign' -headers-git like I do for the cross-compiler-rt package here? (compiler-rt only seems to need the headers, thankfully, as it is a static library. Don't know about the sanitizers, haven't tried to do those for this cross compilation thing yet).

[mati865]

Right, forgot about mingw-w64. That would make it even harder than current approach...