winpthreads: mutex.c: update `pthread_mutex_init` function

Use `pthread_mutexattr_t` value passed to `pthread_mutex_init` directly,
interpreted as `WinpthreadsMutexAttributes` union, instead of retrieving
attributes by calling `pthread_mutexattr_get*` functions; these functions
are still called to validate passed `pthread_mutexattr_t` object.

Simplify `WinpthreadsMutexGet` function by using `WinpthreadsMutexAttributes`
union directly instead of manipulating `pthread_mutexattr_t` object.

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