Try to fix the libcxx build with mingw64

Summary:
mingw64 has lots of default libs that libc++ and its test programs
should link against.

With this patch, cmake now runs successfully with GCC on Windows.

Reviewers: mati865, EricWF

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D31518

llvm-svn: 299144