gdbserver/linux-x86: make is_64bit_tdesc accept thread as a parameter

gdbserver/linux-x86: make is_64bit_tdesc accept thread as a parameter

ps_get_thread_area receives as a parameter the lwpid it must work on.
It then calls is_64bit_tdesc, which uses the current_thread as the
thread to work on.  However, it is not said that both are the same.

This became a problem when working in a following patch that makes
find_one_thread switch to a process but to no thread (current_thread ==
nullptr).  When libthread_db needed to get the thread area,
is_64bit_tdesc would try to get the regcache of a nullptr thread.

Fix that by making is_64bit_tdesc accept the thread to work on as a
parameter.  Find the right thread from the context, when possible (when
we know the lwpid to work on).  Otherwise, pass "current_thread", to
retain the existing behavior.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I44394d6be92392fa28de71982fd04517ce8a3007