Make gdb/linux-nat.c consider a waitstatus pending on the infrun side

Make gdb/linux-nat.c consider a waitstatus pending on the infrun side

Working on the fix for gdb/19828, I saw
gdb.threads/attach-many-short-lived-threads.exp fail once in an
unusual way.  Unfortunately I didn't keep debug logs, but it's an
issue similar to what's been fixed in remote.c a while ago --
linux-nat.c was not fetching the pending status from the right place.

gdb/ChangeLog:
2016-05-24  Pedro Alves  <palves@redhat.com>

	PR gdb/19828
	* linux-nat.c (get_pending_status): If the thread reported the
	event to the core and it's pending, use the pending status signal
	number.