Share get_image_name between gdb and gdbserver

Share get_image_name between gdb and gdbserver

This moves get_image_name to nat/windows-nat.c so that it can be
shared between gdb and gdbserver.

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

	* windows-nat.c (get_image_name): Move to nat/windows-nat.c.
	(handle_load_dll): Update.
	* nat/windows-nat.c (get_image_name): Move from windows-nat.c.

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

	* win32-low.c (get_image_name): Remove.
	(handle_load_dll): Update.