[libcxx] Implement semaphores for windows

Also add WIN32_LEAN_AND_MEAN before including windows.h, for consistency
with other sources.

Differential Revision: https://reviews.llvm.org/D97539

GitOrigin-RevId: 1773eec6928f4e37b377e23b84d7a2a07d0d1d0d
2 files changed