winpthreads: mutex.c: replace `is_static_initializer` function with a macro

This is consistent with other source files.
Document valid values for a statically initialized `pthread_mutex_t` object.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>