gdb: optimize check for resumed threads with pending wait status in...

gdb: optimize check for resumed threads with pending wait status in maybe_set_commit_resumed_all_targets

Consider a test case where many threads (thousands) keep hitting a
breakpoint whose condition evaluates to false.
maybe_set_commit_resumed_all_targets is called at each handled event,
when the scoped_disable_commit_resumed object in fetch_inferior_event is
reset_and_commit-ed.  One particularly expensive check in there is
whether the target has at least one resumed thread with a pending wait
status (in which case, we don't want to commit the resumed threads, as
we want to consume this status first).  It is currently implemented as
walking all threads of the target.

Since we now maintain a per-target list of resumed threads with pending
status, we can do this check efficiently, by checking whether that list
is empty or not.

Add the process_stratum_target::has_resumed_with_pending_wait_status
method for this, and use it in maybe_set_commit_resumed_all_targets.

Change-Id: Ia1595baa1b358338f94fc3cb3af7f27092dad5b6