threadStackOverflow: check whether stack squeezing released some stack (#3677)
In a stack overflow situation, stack squeezing may reduce the stack size, but we don't know whether it has been reduced enough for the stack check to succeed if we try again. Fortunately stack squeezing is idempotent, so all we need to do is record whether *any* squeezing happened. If we are at the stack's absolute -K limit, and stack squeezing happened, then we try running the thread again. We also want to avoid enlarging the stack if squeezing has already released some of it. However, we don't want to get into a pathalogical situation where a thread has a nearly full stack (near its current limit, but not near the absolute -K limit), keeps allocating a little bit, squeezing removes a little bit, and then it runs again. So to avoid this, if we squeezed *and* there is still less than BLOCK_SIZE_W words free, then we enlarge the stack anyway.