gdb.base/interrupt.exp: Use send_inferior/$inferior_spawn_id

gdb.base/interrupt.exp: Use send_inferior/$inferior_spawn_id

The gdb.base/interrupt.exp test is important for testing system call
restarting, but because it depends on inferior I/O, it ends up skipped
against gdbserver.  This patch adjusts the test to use send_inferior
and $inferior_spawn_id so it works against GDBserver.

gdb/testsuite/ChangeLog:
2015-04-07  Pedro Alves  <palves@redhat.com>

	* gdb.base/interrupt.exp: Don't skip if $inferior_spawn_id !=
	$gdb_spawn_id.  Use send_inferior and $inferior_spawn_id to
	interact with inferior program.