linux_nat.c: Mark new thread running even if momentarily pausing

linux_nat.c: Mark new thread running even if momentarily pausing

My all-stop-on-top-of-non-stop series manages to trip on a bug in the
linux-nat.c backend while running the testsuite.  If a thread is
discovered while threads are being momentarily paused (without the
core's intervention), the thread ends up stuck in THREAD_STOPPED
state, even though from the user's perspective, the thread is running
even while it is paused.

From inspection, in the current sources, this can happen if we call
stop_and_resume_callback, though there's no way to test that with
current Linux kernels.

(While trying to come up with test to exercise this, I stumbled on:
  https://sourceware.org/ml/gdb-patches/2015-03/msg00850.html

... which does include a non-trivial test, so I think I can still
claim I come out net positive. :-) )

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-04-01  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_handle_extended_wait): Always call set_running.