srw_mutex_impl<spinloop>::wait_and_lock(): Invoke srw_pause() and
reload the lock word on each loop. Thanks to Mark Callaghan for
suggesting this.

ssux_lock_impl<spinloop>::rd_wait(): Actually implement a spin loop
on the rw-lock component without blocking on the mutex component.
If there is a conflict with wr_lock(), wait for writer.lock to be
released without actually acquiring it.

Reviewed by: Debarun Banerjee
Tested by: Matthias Leich