Fix mi-nonstop.exp with extended-remote

Testing with "maint set target-non-stop on" makes mi-nonstop.exp run
with the extended-remote board.  That reveals that mi-nonstop.exp is
using the wrong predicate to check for "using remote protocol".

This is not visible today because non-stop tests all fail to run with
extended-remote board, because they spawn gdb and then do "set
non-stop on".  However, with that board, gdb connects to the gdbserver
from within mi_gdb_start, and changing non-stop when already connected
doesn't work.  Fix that by instead enabling non-stop mode on gdb's
command line.

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

	* gdb.mi/mi-nonstop.exp: Append "set non-stop on" to GDBFLAGS
	instead of issuing "-gdb-set non-stop 1" after starting gdb.
	Use mi_is_target_remote instead of checking "is_remote target".
	* lib/gdb.exp (gdb_is_target_remote): Rename to ...
	(gdb_is_target_remote_prompt): ... this, and add 'prompt_regexp'
	parameter.
	(gdb_is_target_remote): Reimplement.
	* lib/mi-support.exp (mi_is_target_remote): New procedure.