fix async-cancel-safety of pthread_cancel

fix async-cancel-safety of pthread_cancel

the previous commit addressing async-signal-safety issues around
pthread_kill did not fully fix pthread_cancel, which is also required
(albeit rather irrationally) to be async-cancel-safe.

without blocking implementation-internal signals, it's possible that,
when async cancellation is enabled, a cancel signal sent by another
thread interrupts pthread_kill while the killlock for a targeted
thread is held. as a result, the calling thread will terminate due to
cancellation without ever unlocking the targeted thread's killlock,
and thus the targeted thread will be unable to exit.