Tweak gdb.base/corefile.exp is_remote check

1. Otherwise, when we make native-gdbserver board no longer is_remote,
   we get:

  Running .../src/gdb/testsuite/gdb.base/corefile.exp ...
  ERROR: tcl error sourcing .../src/gdb/testsuite/gdb.base/corefile.exp.
  ERROR: gdbserver does not support attach 9327 without extended-remote
      while executing
  "error "gdbserver does not support $command without extended-remote""

  That's fixed by using can_spawn_for_attach instead.

2. The gdb_protocol check fixes this current problem with
   --target_board=extended-remote-gdbserver:

     Running .../src/gdb/testsuite/gdb.base/corefile.exp ...
     FAIL: gdb.base/corefile.exp: run: with core
     FAIL: gdb.base/corefile.exp: run: core file is cleared
     FAIL: gdb.base/corefile.exp: attach: with core
     FAIL: gdb.base/corefile.exp: attach: core file is cleared

   gdb.log:
     (...)
     attach 10859
     Don't know how to attach.  Try "help target".
     (...)

The fix for #2 alone would fix #1 too, but can_spawn_for_attach
expresses the requirement directly, so I still left it there.

gdb/testsuite/ChangeLog:
2017-10-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/corefile.exp (corefile_test_run): Skip if gdb_protocol
	is set.
	(corefile_test_attach): Likewise.  Check can_spawn_for_attach
	instead of is_remote.