Make dprintf-non-stop.exp cope with remote testing

Testing with the extended-remote board with "maint set target-non-stop
on" shows a dprintf-non-stop.exp regression.  The issue is simply that
the test is expecting output that is only valid for the native target:

 native:

  [process 8676] #1 stopped.

 remote:

  [Thread 8900.8900] #1 stopped.

In order to expose this without "maint set target-non-stop on", this
restarts gdb with non-stop mode already enabled.

gdb/testsuite/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* gdb.base/dprintf-non-stop.exp: Use build_executable instead of
	prepare_for_testing.  Start gdb with "set non-stop on" appended to
	GDBFLAGS.  Lax expected stop output.