Share some Windows-related globals

Share some Windows-related globals

This moves some Windows-related globals into nat/windows-nat.c,
sharing them between gdb and gdbserver.

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

	* windows-nat.c (current_process_handle, current_process_id)
	(main_thread_id, last_sig, current_event, last_wait_event)
	(current_windows_thread, desired_stop_thread_id, pending_stops)
	(struct pending_stop, siginfo_er): Move to nat/windows-nat.c.
	(display_selectors, fake_create_process)
	(get_windows_debug_event): Update.
	* nat/windows-nat.h (current_process_handle, current_process_id)
	(main_thread_id, last_sig, current_event, last_wait_event)
	(current_windows_thread, desired_stop_thread_id, pending_stops)
	(struct pending_stop, siginfo_er): Move from windows-nat.c.
	* nat/windows-nat.c (current_process_handle, current_process_id)
	(main_thread_id, last_sig, current_event, last_wait_event)
	(current_windows_thread, desired_stop_thread_id, pending_stops)
	(siginfo_er): New globals.  Move from windows-nat.c.

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

	* win32-low.c (current_process_handle, current_process_id)
	(main_thread_id, last_sig, current_event, siginfo_er): Move to
	nat/windows-nat.c.