[sgen] Initialize stack_start to 0 when a thread is signaled for stw

[sgen] Initialize stack_start to 0 when a thread is signaled for stw

If we have an invalid SP in the suspended thread, we don't set stack_start in the thread info, expecting it to be 0 in order to signal a resume and suspend retry to the suspender thread. If the thread was already restarted before (from a different reason like IP in critical section), it would have a nonzero stack_start from the previous suspend, leading to an assertion in the previously described case.

This is also what we do on mach.