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.

gdb/utils.c

@@ -691,6 +691,15 @@ malloc_failure (long size)
     }
 }
 
+/* See common/errors.h.  */
+
+void
+flush_streams ()
+{
+  gdb_stdout->flush ();
+  gdb_stderr->flush ();
+}
+
 /* My replacement for the read system call.
    Used like `read' but keeps going if `read' returns too soon.  */