Use debug_printf in windows-nat.c

While debugging a bug on Windows, I noticed that windows-nat.c is not
sending its debugging output to gdb_stdlog.  This is unfortunate
because it means that "set logging debugredirect" doesn't work
properly.

This patch fixes the problem by changing windows-nat.c to use
debug_printf.

Note that get_windows_debug_event also writes one debugging message
unconditionally.  It isn't clear to me if this really ought to use
DEBUG_EVENTS or not, since it seems like perhaps it is intended to
note an unexpected event occurring.  So, I didn't change this.

I'm checking this in.

gdb/ChangeLog
2020-04-15  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (DEBUG_EXEC, DEBUG_EVENTS, DEBUG_MEM)
	(DEBUG_EXCEPT): Use debug_printf.