remote: stop reason and watchpoint data address per thread

Running local-watch-wrong-thread.exp with "maint set target-non-stop
on" exposes that gdb/remote.c only records whether the target stopped
for a breakpoint/watchpoint plus the watchpoint data address *for the
last reported remote event*.  But in non-stop mode, we need to keep
that info per-thread, as each thread can end up with its own
last-status pending.

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

	* remote.c (struct remote_state) <remote_watch_data_address,
	stop_reason>: Delete fields.
	(struct private_thread_info) <stop_reason, watch_data_address>:
	New fields.
	(resume_clear_thread_private_info): New function.
	(append_pending_thread_resumptions): Call it.
	(remote_resume): Clear all threads' private info.
	(process_stop_reply): Adjust.
	(remote_wait_as): Don't reference remote_state's stop_reason
	field.
	(remote_stopped_by_sw_breakpoint)
	(remote_stopped_by_hw_breakpoint, remote_stopped_by_watchpoint)
	(remote_stopped_data_address): Adjust to refer get data from the
	current thread.