sigprocmask: On mingw, avoid dependency on libwinpthread.

sigprocmask: On mingw, avoid dependency on libwinpthread.

* lib/sigprocmask.c: Include windows-tls.h, windows-once.h.
(thread_local): Remove macro.
(struct per_thread): New type.
(tls_key): New variable.
(keys_init): New function.
(keys_init_once): New variable.
(get_per_thread): New function.
(per_thread_singleton): New variable.
(blocked_set, pending_array): Remove variables.
(sigpending, override_handler, pthread_sigmask, _gl_raise_SIGPIPE):
Invoke get_per_thread, to access blocked_set or pending_array.
* modules/sigprocmask (Depends-on): Add windows-once, windows-tls.