alpha: take SIGPENDING/NOTIFY_RESUME loop into signal.c

Turn the slow side of work_pending into C function, including all
the looping.  What we get out of that:
	* we do _not_ call get_signal_to_deliver() with IRQs disabled
anymore
	* no need to save/restore volatiles on each pass if there
turns to be more than one (unlikely, but still)
	* all double-restart prevention is in C now.
	* glue gets simpler.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>