signal: Add block_sigmask() for adding sigmask to current->blocked

signal: Add block_sigmask() for adding sigmask to current->blocked

This patch abstracts the code sequence for adding a signal handler's
sa_mask to current->blocked because the sequence is identical for all
architectures. Furthermore, in the past some architectures actually
got this code wrong, so introduce a wrapper that all architectures can
use.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>