cargo: Use the right LLVM tools and arguments for MSVC cross-compilation

[MarijnS95]

xbuild uses the headers from xwin to support cross-compiling, but doesn't fully replicate the other setup steps that it proposes to provide a valid and working cross-compilation setup to MSVC. We need to make the following changes:

• clang-cl must be used (a clang driver with cl.exe interface), as all native projects will call CC/CXX with flags that are only compatible with cl.exe;

• -I does not seem to set up the system headers correctly, for this -imsvc should be used;

• Remove -Clink-arg=-fuse-ld=lld-link because we have already selected a linker for Rust with the LINKER env var (see warning below). We might instead want to set it in C(XX)FLAGS so that when a native crate uses CC/CXX to link libraries, it can use lld-link instead of (probably, like cargo/rustc when LINKER is unsed) using link.exe by default;

• Unset forced static CRT and stdlib=libc++ selection, as these don't appear to ever be needed, and cause some builds to fail (when crates are changing these config options themselves?):

  = note: rust-lld: warning: ignoring unknown argument '-fuse-ld=lld-link'
          rust-lld: error: /failifmismatch: mismatch detected for 'RuntimeLibrary':
          >>> libbasis_universal_sys-4f5abd4a27e6aa13.rlib(basisu_resample_filters.o) has value MT_StaticRelease
          >>> libintel_tex_2-6d9bd3438efd06d9.rlib(ispc_texcomp_astc.o) has value MD_DynamicRelease