Fix gdb.base/interrupt.exp racy fail against gdbserver

Fix gdb.base/interrupt.exp racy fail against gdbserver

Currently, against gdbserver, interrupt.exp occasionaly fails like
this:

 ERROR: Process no longer exists
 UNRESOLVED: gdb.base/interrupt.exp: send end of file

The problem is that we see gdbserver exiting before we match gdb's
output:

 expect: does "\r\n\r\nChild exited with status 0\r\nGDBserver exiting\r\n" (spawn_id exp8) match regular expression "end of file"? Gate "end of file"? gate=no
 expect: read eof
 expect: set expect_out(spawn_id) "exp8"
 expect: set expect_out(buffer) "\r\n\r\nChild exited with status 0\r\nGDBserver exiting\r\n"

Fix this by removing $inferior_spawn_id from the set of spawn ids
expect is watching as soon as we see the "end of file" string out of
the inferior spawn id, using an indirect spawn id list.

Tested on x86-64 Fedora 20, native and gdbserver (both target remote
and extended-remote).

gdb/testsuite/ChangeLog:
2015-04-23  Pedro Alves  <palves@redhat.com>

	* gdb.base/interrupt.exp: Use an indirect spawn id list holding
	$inferior_spawn_id instead of $inferior_spawn_id directly.  On
	"end of file", remove $inferior_spawn_id from the indirect list.