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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+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.
+
 2015-07-29  Sergio Durigan Junior  <sergiodj@redhat.com>
 
 	* gdb.threads/attach-into-signal.exp (corefunc): Use

gdb/testsuite/gdb.base/multi-forks.exp

@@ -50,6 +50,7 @@ set exit_bp_loc [gdb_get_line_number "Set exit breakpoint here."]
 
 proc continue_to_exit_bp_loc {} {
     global exit_bp_loc decimal gdb_prompt
+    global inferior_spawn_id gdb_spawn_id
 
     gdb_breakpoint $exit_bp_loc
 
@@ -70,6 +71,7 @@ proc continue_to_exit_bp_loc {} {
 	# first in the script that occurs anywhere in the input, so that
 	# we don't skip anything.
 	gdb_expect {
+	    -i "$inferior_spawn_id $gdb_spawn_id"
 	    -re "($decimal done)|(Breakpoint)|($gdb_prompt)" {
 		if {[info exists expect_out(1,string)]} {
 		    incr seen_done