util: Add a Win32 futex implementation
The goal here is to simplify !17122 (merged) and make _SIMPLE_MTX_INITIALIZER_NP
portable by making it never map to _MTX_INITIALIZER_NP
on platforms where it's not legal to initialize mutexes that way. This might have a side benefit of speeding up the Windows build of Mesa by using futexes instead of critical sections.