RFR: Locked allocation

[Roland Westrelin]

> One issue was that Mutex was expecting the thread in VM, unless it's
> rank special. We can only be in VM when we have a non-leaf call at the
> write barrier.
>
> If I make the lock ranked 'special' I run into asserts that check
> correct lock ordering. We need to allocate stuff when evacuating roots,
> and this is holding the CodeCache_lock which is also ranked 'special'
> etc pp.

Ok. Thanks.

Roland.