Fetch registers from correct thread in ravenscar-thread.c

Fabien also noticed that gdb would not report a stop correctly when
using Ravenscar.  This patch fixes the bug by making a few changes:

* ravenscar_thread_target::wait now updates the inferior ptid before
  updating the thread list.  This ensures that a new thread is
  correctly associated with the underlying CPU.

* The fetch_registers, store_registers, and prepare_to_store methods
  now save and restore the regcache's ptid before doing the operation
  on the underlying live thread.  This ensures that gdb informs the
  remote of a thread it knows about, as opposed to using a Ravenscar
  thread, which probably will not be recognized.

gdb/ChangeLog
2020-08-07  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::wait): Call
	update_inferior_ptid before update_thread_list.
	(temporarily_change_regcache_ptid): New class.
	(ravenscar_thread_target::fetch_registers)
	(ravenscar_thread_target::store_registers)
	(ravenscar_thread_target::prepare_to_store): Use base thread when
	forwarding operation.