winpthreads: mutex.c: remove static function `__pthread_mutex_timedlock`

In 87281623c635c264cb128d4fd9af54c7a4705ae6, function `pthread_mutex_timedlock`
was renamed to `__pthread_mutex_timedlock` and modified to always use
`struct _timespec64`; it was used as a common implementation for
`pthread_mutex_timedlock{32|64}` functions.

Move code from `__pthread_mutex_timedlock` to `pthread_mutex_timedlock64` and
call `pthread_mutex_timedlock64` from `pthread_mutex_timedlock32`.

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