winpthreads: mutex.c: implement `pthread_mutexattr_{set,get}robust` functions
This change is a preparation step for implementing robust mutexes.
Add new test `pthread_mutex/attr-robust.c`.
Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>