Errors when host clang defaults to libc++ and compiler-rt

[jprjr]

Hi there - I built a Docker container with clang set to default to using libc++ and compiler-rt, so when I run wclang, it too tries to use libc++ and compiler-rt - I'm not 100% positive if these are usable on Windows yet.

Running x86_64-w64-mingw32-clang++ with -rtlib=platform and -stdlib=platform restores the default behavior. I would love to be able to specify a default rtlib and stdlib at compile-time in cmake