Backport Boehm GC changes for Winpthreads support.

Make our changes to support Winpthreads match those of upstream
bdwgc commit 94b85eea2e2713c7a6071f1965b1e98e645e65a7

boehm_gc/include/private/gc_locks.h:
boehm_gc/win32_threads.c:
	As above.