Introduce and use flush_streams

Introduce and use flush_streams

Code in gdbsupport can't call gdb_flush, so this introduces a new
"flush_streams" function that must be supplied by the client.

Note that the similar gdb_flush_out_err exists, but it isn't defined
in quite the same way, so it wasn't clear to me whether the two could
be merged.

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

	* utils.c (flush_streams): New function.
	* event-loop.c (gdb_wait_for_event): Call flush_streams.

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

	* errors.h (flush_streams): Declare.