Clean up SuspendibleThreadSet::_suspend_all

The SuspendibleThreadSet::_suspend_all variable is read and set concurrently, yet relies only on volatiles in accessors. They should use Atomic instead. Also, the _suspend_all getter is called suspend_all(), but since this function doesn't suspend anything, the name looks really confusing where it is, and there is an alias should_yield() that just calls suspend_all(), and better explains what the code does. This CR aims to remove suspend_all() and have a single getter, being should_yield (which reads better everywhere it is used), and make sure the accessors do use Atomic.