Move start_event_loop out of event-loop.c

Move start_event_loop out of event-loop.c

A subsequent patch is going to move event-loop.c to gdbsupport.  In a
review of an earlier version of this series, Pedro pointed out that
the resulting code would be cleaner if start_event_loop were not
shared -- because gdb and gdbserver have some different needs here --
and so this moves start_event_loop to main.c.  Because the only caller
is there, it is also now static.

gdb/ChangeLog
2020-04-13  Tom Tromey  <tom@tromey.com>

	* event-loop.h (start_event_loop): Don't declare.
	* event-loop.c (start_event_loop): Move...
	* main.c (start_event_loop): ...here.  Now static.