[libcxx] [test] Detect mingw-w64 headers compatible with C++ module builds (#92893)

This fixes running the tests/CI with a newer mingw toolchain that has
been fixed to work with building libc++ as a module.

NOKEYCHECK=True
GitOrigin-RevId: 337e633bb75640bb6e04eb874c7114dfac6fa754
1 file changed