Use debug_prefixed_printf_cond in windows-nat.c

Use debug_prefixed_printf_cond in windows-nat.c

This changes windows-nat.c and nat/windows-nat.c to use the new
debug_prefixed_printf_cond facility.  I tried this out on a Windows
build and I think it makes the output look a little nicer.

2021-01-26  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (DEBUG_EXEC, DEBUG_EVENTS, DEBUG_MEM)
	(DEBUG_EXCEPT): Use debug_prefixed_printf_cond.
	(windows_init_thread_list, windows_nat::handle_load_dll)
	(windows_nat::handle_unload_dll, windows_nat_target::resume)
	(windows_nat_target::resume)
	(windows_nat_target::get_windows_debug_event)
	(windows_nat_target::interrupt, windows_xfer_memory)
	(windows_nat_target::close): Update.
	* nat/windows-nat.c (DEBUG_EVENTS): Use
	debug_prefixed_printf_cond.
	(matching_pending_stop, fetch_pending_stop)
	(continue_last_debug_event): Update.