[vcpkg-scripts][gmp][nettle] Use VSCLANG

[dg0yt]

Cherry-picked and revised from #31228:
Use clang from Visual Studio for assembler or fail with a clear error message. Don't download llvm

For #28912 and several similar reports.

[Neumann-A]

I am against this. There is no reason to force clang from VS.

[dg0yt]

I am against this. There is no reason to force clang from VS.

Only against not accepting another installed clang (via LLVMInstallDir, or even from downloads/tools),
or against not downloading LLVM?

[Neumann-A]

I am against forcing clang from VS. So if it is not installed you could print a info message and then go get LLVM. LLVMInstallDir is not passed through so you never see that variable in normal triplets.

I mean the current behavior is already -> get clang -> if installed in VS use that -> otherwise download llvm and use that. (Third way: Have the variable set in the triplet and directly feed it.)