Fix ravenscar-thread.c for multi-target

Fix ravenscar-thread.c for multi-target

ravenscar-thread.c needed a change to adapt to multi-target:
ravenscar_thread_target::mourn_inferior called the mourn_inferior
method on the target beneat -- but when the target beneath was the
remote target, this resulted in the ravenscar target being deleted.

Switching the order of the calls to unpush_target and the beneath's
mourn_inferior fixes this problem.

gdb/ChangeLog
2020-01-31  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::mourn_inferior):
	Call beneath target's mourn_inferior after unpushing.

Change-Id: Ia80380515c403adc40505a6b3420c9cb35754370