Exception safety in ravenscar-thread.c

Exception safety in ravenscar-thread.c

This changes some code in ravenscar-thread.c to use scoped_restore.  I
am not sure if it matters in practice, but this makes these methods
exception-safe in case the methods lower in the target stack can
throw.

gdb/ChangeLog
2019-02-15  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::stopped_by_sw_breakpoint)
	(ravenscar_thread_target::stopped_by_hw_breakpoint)
	(ravenscar_thread_target::stopped_by_watchpoint)
	(ravenscar_thread_target::stopped_data_address)
	(ravenscar_thread_target::core_of_thread): Use scoped_restore.