Share DLL code between gdb and gdbserver

This moves the new DLL-loading code into nat/windows-nat.c, and
changes both gdb and gdbserver to use the shared code.  One
client-provided callback, handle_load_dll, is changed to allow the
code to be shared.  This callback was actually never called from
nat/windows-nat.c; maybe I had planned to share more here and then
didn't finish... I'm not sure.

gdb/ChangeLog
2021-04-30  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_nat::handle_load_dll): Update.
	(windows_nat_target::get_windows_debug_event): Call
	dll_loaded_event.
	(windows_add_all_dlls, windows_add_dll): Move to
	nat/windows-nat.c.
	* nat/windows-nat.h (handle_load_dll): Change parameters.
	(dll_loaded_event, windows_add_all_dlls): Declare.
	* nat/windows-nat.c (windows_add_dll, windows_add_all_dlls): Move
	from windows-nat.c.
	(dll_loaded_event): New function.

gdbserver/ChangeLog
2021-04-30  Tom Tromey  <tromey@adacore.com>

	* win32-low.cc (do_initial_child_stuff): Update.
	(windows_nat::handle_load_dll): Rename from win32_add_one_solib.
	Change parameter type.
	(win32_add_dll, win32_add_all_dlls)
	(windows_nat::handle_load_dll): Remove.
	(get_child_debug_event): Call dll_loaded_event.