nptl: Check if thread is already terminated in sigcancel_handler (BZ 32782)

nptl: Check if thread is already terminated in sigcancel_handler (BZ 32782)

The SIGCANCEL signal handler should not issue __syscall_do_cancel,
which calls __do_cancel and __pthread_unwind, if the cancellation
is already in proces (and libgcc unwind is not reentrant).  Any
cancellation signal received after is ignored.

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

Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Florian Weimer <fweimer@redhat.com>