windows: Use ptid from regcache in register fetch/store

windows: Use ptid from regcache in register fetch/store

Use the ptid from the regcache so we don't depend on the current value
of the inferior_ptid global.

Also, change how the current thread is passed to sub-functions.  The
windows_fetch_inferior_registers function sets current_thread then calls
do_windows_fetch_inferior_registers, which reads current_thread.  This
very much looks like passing a parameter through a global variable.  I
think it would be more straightforward to pass the thread as a
parameter.

gdb/ChangeLog:

	* windows-nat.c (do_windows_fetch_inferior_registers): Add
	windows_thread_info parameter and use it instead of
	current_thread.
	(windows_fetch_inferior_registers): Don't set current_thread,
	pass the thread to do_windows_fetch_inferior_registers.  Use
	ptid from regcache instead of inferior_ptid.
	(do_windows_store_inferior_registers): Add windows_thread_info
	parameter and use it instead of current_thread.
	(windows_store_inferior_registers): Don't set current_thread,
	pass the thread to do_windows_store_inferior_registers.  Use
	ptid from regcache instead of inferior_ptid.