Set inferior_ptid in ravenscar_thread_target::update_thread_list

Commit 2da4b788 ("Don't write to inferior_ptid in
ravenscar-thread.c") caused a Ravenscar regression (which, FWIW, is
understandable because Ravenscar is difficult to test).  Namely,
ravenscar_thread_target::update_thread_list calls
iterate_over_live_ada_tasks, which calls ada_build_task_list, which
uses target_has_stack -- which relies on inferior_ptid.

This patch changes update_thread_list to ensure that inferior_ptid is
set before making this call.  This avoids various failures on
Ravenscar targets.

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

	* ravenscar-thread.c (update_thread_list): Set inferior_ptid.