gdbserver/linux: Always wake up event loop after resume

Running killed-outside.exp in with "maint set target-non-stop on"
hangs currently.  This test has the inferior process die with a
SIGKILL while stopped.  gdbserver gets a SIGCHLD and reacts by
retrieveing the SIGKILL events out of waitpid.  But because the
process is not resumed from GDB's perspective, the event is left
pending.  When GDB resumes the process afterwards, the process is not
really resumed because it already has the event pending.  But nothing
wakes up the event loop to consume the event.

Handle this in the same way nat/linux-nat.c:linux_nat_resume handles
this.

gdb/gdbserver/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_resume): Wake up the event loop before
	returning.