gdb, btrace: set wait status to ignore if nothing is moving

gdb, btrace: set wait status to ignore if nothing is moving

When record_btrace::wait() is called and no threads are moving, we set the
wait status to no_resumed.  Change that to ignore.

This helps with enabling per-inferior run-control for the record btrace
target as it avoids breaking out of do_target_wait() too early with
no_resumed when there would have been an event on another thread.