[libcxx] [test] Add a MinGW target

This can't easily be autodetected (unless LIBCXX_TARGET_TRIPLE is
specified, or unless we query what the compiler's default target is,
which only is supported by clang), but can be chosen manually via
LIBCXX_TARGET_INFO.

This chooses mingw style lib naming, and uses -nostdlibc++ instead
of -nodefaultlib -nostdlib (as the latter requires specifying a lot of
details manually - this is done in the cmake config though).

Differential Revision: https://reviews.llvm.org/D97294