Relax Clang frontend variant detection

[silvanshade]

This allows Clang frontend variant detection to work better in more scenarios such as when compiling with cross-compilers with platform prefixes.

In particular this fixes cross-compiles from Linux to FreeBSD for nix packaging.

It would be good to get this in the next patch release but probably isn't critical enough to warrant it's own release since it should only show up when using both Clang and specially named cross-compiler (which is somewhat unusual since Clang already supports multiple targets).

CC @Ericson2314

[BurningEnlightenment]

Would the cross compiling detection automatically work if we used CMAKE_CXX_COMPILER_FRONTEND_VARIANT?

[silvanshade]

Would the cross compiling detection automatically work if we used CMAKE_CXX_COMPILER_FRONTEND_VARIANT?

Yes.

With that in mind I refactored the existing detection code to use CMAKE_CXX_COMPILER_FRONTEND_VARIANT if available and fall back to our re-implementation otherwise.

I also moved the logic into BLAKE3/Utils.cmake which is probably cleaner if you're moving some of the other stuff there in the other PR.