Make gdb.base/multi-forks.exp work with the native-extended-gdbserver board

Make gdb.base/multi-forks.exp work with the native-extended-gdbserver board

Now that we can expect inferior output with the gdbserver boards, this
is all it takes to have the test pass against extended-remote
gdbserver.

Don Breazeal originally wrong something like this:

 https://sourceware.org/ml/gdb-patches/2015-03/msg00506.html

which was what originally inspired the introduction of
$inferior_spawn_id.

gdb/testsuite/ChangeLog:
2015-07-29  Pedro Alves  <palves@redhat.com>
	    Don Breazeal  <donb@codesourcery.com>

	* gdb.base/multi-forks.exp (continue_to_exit_bp_loc): Expect
	output from both inferior_spawn_id and gdb_spawn_id.