Enable gdb.base/inferior-died.exp on is_remote target boards

Enable gdb.base/inferior-died.exp on is_remote target boards

We support follow-fork in the remote protocol nowadays.

Also, the right way to enable non-stop mode is to do it before
connecting, and for use_gdb_stub boards, that means we have to do it
at gdb_load time.  The "modern" pattern for that is to pass non-stop
in GDBFLAGS.

This makes the test pass with --target_board=native-gdbserver.

gdb/testsuite/ChangeLog:
2017-10-12  Pedro Alves  <palves@redhat.com>
	    Simon Marchi <simon.marchi@polymtl.ca>

	* gdb.base/inferior-died.exp: Remove is_remote and isnative
	checks.  Use build_executable + clean_restart instead of
	prepare_for_testing.  Pass "set non-stop on" via GDBFLAGS instead
	of enabling non-stop after starting gdb.