Introduce async-event.[ch]

Introduce async-event.[ch]

This patch splits out some gdb-specific code from event-loop, into new
files async-event.[ch].  Strictly speaking this code could perhaps be
put into gdbsupport/, but because gdbserver does not currently use it,
it seemed better, for size reasons, to split it out.

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

	* tui/tui-win.c: Include async-event.h.
	* remote.c: Include async-event.h.
	* remote-notif.c: Include async-event.h.
	* record-full.c: Include async-event.h.
	* record-btrace.c: Include async-event.h.
	* infrun.c: Include async-event.h.
	* event-top.c: Include async-event.h.
	* event-loop.h: Move some declarations to async-event.h.
	* event-loop.c: Don't include ser-event.h or top.h.  Move some
	code to async-event.c.
	* async-event.h: New file.
	* async-event.c: New file.
	* Makefile.in (COMMON_SFILES): Add async-event.c.
	(HFILES_NO_SRCDIR): Add async-event.h.