nptl: Use pthread_kill on pthread_cancel

It consolidates the tgkill call and it is the first step of making
pthread_cancel async-signal-safe.  It also fix a possible issue
where the 'struct pthread' tid is not read atomically, which might
send an invalid cancellation signal (similar to what
db988e50 fixed for pthread_join).

Checked on x86_64-linux-gnu and aarch64-linux-gnu.