package mingw-w64-clang-x86_64-rust should target to x86_64-pc-windows-gnullvm

[ArchGuyWu]

As there's only llvm-based toolchain in ucrt64 repo ,the target x86_64-pc-windows-gnu is not the proper one ,and it doesn't work even with package mingw-w64-clang-x86_64-gcc-compat installed.

[1480c1]

https://doc.rust-lang.org/rustc/platform-support/pc-windows-gnullvm.html

should probably ping @mati865 for his opinion

[jeremyd2019]

I think that's being worked on, #13278 is related

[mati865]

As there's only llvm-based toolchain in ucrt64 repo

UCRT64 repo is GCC/Binutils based, CLANG64 is LLVM based.

the target x86_64-pc-windows-gnu is not the proper one ,and it doesn't work

It works fine because of hacks, we have compiled multiple packages with it: https://packages.msys2.org/package/mingw-w64-clang-x86_64-rust

mingw-w64-clang-x86_64-rust should target to x86_64-pc-windows-gnullvm

That is long-term goal but:

1. Getting it to build with pacman under MSYS2 has been troublesome and so far it's even more hacky than using regular *-pc-windows-gnu triples.
2. Ecosystem is not ready yet. Because of the dependencies, migrating right now would break building half of the packages (or more) that we already build with Rust for CLANG64.