winpthreads: mutex.c: rename `WinpthreadsTimedWait` -> `WinpthreadsStalledMutexTimedLock`

This change is a preparation step for implementing robust mutexes.

Function `WinpthreadsTimedWait` implements common wait logic for stalled
mutexes; rename it to `WinpthreadsStalledMutexTimedLock` to reflect that.

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