Use find_thread_in_random in select_event_lwp

Use find_thread_in_random in select_event_lwp

I noticed that find_thread_in_random duplicates the code in
find_thread_in_random, so this patch changes the latter to use the
former.

There are two other spots in gdb that do this, but to unify all of
them would require switching some code from using the "iterate over"
idiom to using iterators.

Another possible improvement is that find_thread_in_random could be
made single-pass using reservoir sampling.

Tested by the buildbot.

gdb/gdbserver/ChangeLog
2019-04-09  Tom Tromey  <tromey@adacore.com>

	* linux-low.c (select_event_lwp): Use find_thread_in_random.