GThread posix: switch to Windows ABI

Modify the POSIX implementation of the synchronisation primatives to use
the same ABI as Windows: one pointer for each type.

This frees us from having to #include <pthread.h> and avoids the problem
with pthread_rwlock_t not being defined under certain compiler defines.

A few more changes are expected to the ABI -- they will be committed
separately.

https://bugzilla.gnome.org/show_bug.cgi?id=659866