[libc++] Build thread_win32.cpp only if LIBCXX_HAS_PTHREAD_API is not set

This allows building libc++ against winpthreads from mingw-w64 to support
operating systems older than Windows 7. The remaining libc++ code already
supports `WIN32` with `LIBCXX_HAS_PTHREAD_API`.

Note that there is also the older "pthreads-win32". However, that support
library implements `pthread_t` as a struct, which violates the libc++
assumption that `pthread_t` is always a scalar and can be compared,
ordered, and set to zero.

Differential Revision: https://reviews.llvm.org/D96339