This reduces the latency between a context-switch being triggered and
the thread returning to the scheduler, which in turn should reduce the
cost of the GC barrier when there are many cores.

We still retain the old context_switch flag which is checked at the
end of each block of allocation.  The idea is that setting HpLim may
fail if the the target thread is modifying HpLim at the same time; the
context_switch flag is a fallback.  It also allows us to "context
switch soon" without forcing an immediate switch, which can be costly.