Fix gdb.mi/mi-nsmoribund.exp timeouts

Fix gdb.mi/mi-nsmoribund.exp timeouts

The PPC64 buildbot has been showing timeouts in mi-nsmoribund.exp,
like this:

 (...)
 -thread-info
 FAIL: gdb.mi/mi-nsmoribund.exp: thread state: all running except the breakpoint thread (timeout)

... and I can reproduce this on gcc110 (PPC64) on the gcc compile
farm.

That is, the test sends "-thread-info" to GDB, but GDB never replies
back.

The problem is that these machines are too fast for gdb.  :-)

That test has a few threads running the same tight loop, and
constantly hitting a thread-specific breakpoint that needs to be
stepped over.  If threads trip on breakpoints fast enough that
linux-nat.c's event pipe associated with SIGCHLD is constantly being
written to, even if the stdin file descriptor also has an event to
handle, gdb never gets to it. because linux-nat.c's pipe comes first
in the set of descriptors served by the poll/select code in the event
loop.

Fix this by having the event loop serve file event sources in
round-robin-like fashion, similarly to how its done in
gdb_do_one_event.

Unfortunately, the poll and the select variants each need their own
fixing.

Tested on x86_64 Fedora 20 (poll and select variants), and PPC64
Fedora 18.  Fixes the timeout in the PPC64 machine in the compile farm
that times out without this, and I won't be surprised if it fixes
other random timeouts in other tests.

(gdbserver's copy of the event-loop doesn't need this (yet), as it
still pushes all ready events to an event queue.  That is, it hasn't
had 70b66289 merged yet.  We should really merge both event-loop.c
copies into a single shared file, but that's for another day.)

gdb/ChangeLog:
2015-05-15  Pedro Alves  <palves@redhat.com>
	    Simon Marchi  <simon.marchi@ericsson.com>

	* event-loop.c (gdb_notifier) <next_file_handler,
	next_poll_fds_index>: New fields.
	(get_next_file_handler_to_handle_and_advance): New function.
	(delete_file_handler): If deleting the next file handler to
	handle, advance to the next file handler.
	(gdb_wait_for_event): Bail early if no event fired.  Poll file
	handlers in round-robin fashion.