hle/scheduler: Fix data race in is_context_switch_pending

As reported by tsan, SelectThreads could write to
is_context_switch_pending holding a mutex while SwitchToCurrent reads it
without holding any.

It is assumed that the author didn't want an atomic here, so the code is
reordered so that whenever is_context_switch_pending is read inside
SwitchToContext, the mutex is locked.