gdb/testsuite: fix gdb.gdb/selftest.exp for native-extended-gdbserver

Following commit 4e2a80ba606 ("gdb/testsuite: expect SIGSEGV from top
GDB spawn id"), the next failure I get in gdb.gdb/selftest.exp, using
the native-extended-gdbserver, is:

    (gdb) PASS: gdb.gdb/selftest.exp: send ^C to child process
    signal SIGINT
    Continuing with signal SIGINT.
    FAIL: gdb.gdb/selftest.exp: send SIGINT signal to child process (timeout)

The problem is that in this gdb_test_multiple:

    set description "send SIGINT signal to child process"
    gdb_test_multiple "signal SIGINT" "$description" {
	-re "^signal SIGINT\r\nContinuing with signal SIGINT.\r\nQuit\r\n.* $" {
	    pass "$description"
	}
    }

The "Continuing with signal SIGINT" portion is printed by the top GDB,
while the Quit portion is printed by the bottom GDB.  As the
gdb_test_multiple is written, it expects both the the top GDB's spawn
id.

Fix this by splitting the gdb_test_multiple in two.  The first one
expects the "Continuing with signal SIGINT" from the top GDB.  The
second one expect "Quit"  and the "(xgdb)" prompt from
$inferior_spawn_id.  When debugging natively, this spawn id will be the
same as the top GDB's spawn id, but it's different when debugging with
GDBserver.

Change-Id: I689bd369a041b48f4dc9858d38bf977d09600da2