Change names given to Ravenscar threads

Current a Ravenscar thread is given the same sort of name as a "CPU"
thread; they can only be distinguished by looking at the output of
"info thread".

This patch changes ravenscar-thread.c to distinguish these threads,
like:

    (gdb) continue
    Continuing.
    [New Ravenscar Thread 0x2b910]

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

	* ravenscar-thread.c (ravenscar_thread_target) <extra_thread_info>:
	Remove.
	(ravenscar_thread_target::extra_thread_info): Remove.
	(ravenscar_thread_target::pid_to_str): Mention Ravenscar in result;
	defer to target beneath for non-Ravenscar threads.