Normalize handle_output_debug_string API

Normalize handle_output_debug_string API

This changes gdbserver's implementation of handle_output_debug_string
to have the same calling convention as that of gdb.  This allows for
sharing some more code in a subsequent patch.

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

	* windows-nat.c (windows_nat::handle_output_debug_string):
	Rename.  No longer static.
	* nat/windows-nat.h (handle_output_debug_string): Declare.

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

	* win32-low.c (handle_output_debug_string): Add parameter.  Change
	return type.
	(win32_kill, get_child_debug_event): Update.