Move wait_for_debug_event to nat/windows-nat.c

Move wait_for_debug_event to nat/windows-nat.c

This moves the wait_for_debug_event helper function to
nat/windows-nat.c, and changes gdbserver to use it.
wait_for_debug_event is a wrapper for WaitForDebugEvent that also sets
last_wait_event when appropriate.  This is needed to properly handle
queued stops.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (wait_for_debug_event): Move to
	nat/windows-nat.c.
	* nat/windows-nat.h (wait_for_debug_event): Declare.
	* nat/windows-nat.c (wait_for_debug_event): Move from
	windows-nat.c.  No longer static.

gdbserver/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* win32-low.c (win32_kill, get_child_debug_event): Use
	wait_for_debug_event.