Check lwp_signal_can_be_delivered for enqueue/dequeue pending signals

The enqueue and dequeue signals in linux_resume_one_lwp_throw use one
condition and its inverted one.  This patch is to move the condition
into a function lwp_signal_can_be_delivered, so that the next patch can
change the condition in one place.

gdb/gdbserver:

2016-03-18  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (lwp_signal_can_be_delivered): New function.
	(linux_resume_one_lwp_throw): Use lwp_signal_can_be_delivered.