Bump GC_collect_at_heapsize in GC_add_to_heap. In the normal case it is
recalculated straight after. But in the case where it's called directly
from alloc_mark_stack, it isn't. If the mark stack gets very large,
this can cause GC_should_collect to always return true, and
GC_alloc_large to get stuck in a loop calling GC_collect_or_expand.
Merge pull request #30 from Hoops/loop-in-GC_alloc_large

Loop in GC_alloc_large