Remove CAS from `signal`

[hq6]

Ideally we would like signal to be a blind write, but currently this may induce spurious wakeups in schedulerMainLoop, which would cause the target core to attempt to run a non-existent thread.

One possibility for fixing this is to use some value (all-zeroes) for the function block (storage location for function and arguments of new threads) to indicate that the thread is non-runnable. Under this proposal, schedulerMainLoop will check whether the function block is entirely zeroes after returning from dispatch(). If it is not, then assume it is a spurious wakeup and call continue. Also, on thread exit, before clearing the occupiedAndCount, set the function block to all-zeroes to indicate that the ThreadContext is unoccupied.

This eliminates ThreadContext::UNOCCUPIED and the CAS in signal.