This patch simplify sigpause by remobing the single thread optimization
since it will be handled already by the __sigsuspend call.
Checked on x86_64-linux-gnu.
* sysdeps/posix/sigpause.c (do_sigpause): Remove.
(__sigpause): Rely on __sigsuspend to implement single thread
optimization. Add LIBC_CANCEL_HANDLED for cancellation marking.