Wrap xfer_partial and enable_btrace for Ravenscar

A gdb crash showed that the xfer_partial target method was not wrapped
for Ravenscar.  This caused remote.c to call
remote::set_general_thread with a Ravenscar "fake" ptid, which showed
up later as an event ptid.

I went through all the target methods and looked to see which ones
could call set_general_thread or set_continue_thread (but not
set_general_process, as I think Ravenscar targets aren't
multi-inferior).  This patch wraps the two that I found.

xfer_partial requires special treatment, because it can be called
recursively via get_base_thread_from_ravenscar_task.  To avoid a
recursive call, this patch changes update_thread_list to record all
tasks in the m_cpu_map, and changes get_thread_base_cpu to prefer this
map.  This avoids some memory reads.

It was unclear to me whether enable_btrace really makes sense for
Ravenscar; but at the same time it seemed harmless to add this patch.

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

	* ravenscar-thread.c (xfer_partial, enable_btrace, add_thread):
	New methods.
	(ravenscar_thread_target::get_thread_base_cpu): Check m_cpu_map
	first.
	(ravenscar_thread_target::add_thread): Rename from
	ravenscar_add_thread.
	(ravenscar_thread_target::update_thread_list): Use a lambda.
	(ravenscar_thread_target::xfer_partial): New method.