Switch gdbserver to gdbsupport event loop

Switch gdbserver to gdbsupport event loop

This changes gdbserver to use the gdbserver event loop, removing the
ancient fork.

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

	* server.h (handle_serial_event, handle_target_event): Update.
	* server.c: Don't call initialize_event_loop.
	(keep_processing_events): New global.
	(handle_serial_event): Return void.  Set keep_processing_events.
	(handle_target_event): Return void.
	(start_event_loop): Move from event-loop.c.  Rewrite.
	* remote-utils.c (handle_accept_event): Return void.
	(reset_readchar): Use delete_timer.
	(process_remaining): Return void.
	(reschedule): Use create_timer.
	* event-loop.h: Remove.
	* event-loop.cc: Remove.
	* Makefile.in (OBS): Use gdbsupport/event-loop.o, not event-loop.o.