Reduce memory pressure on system malloc when allocating large items on the GC heap

Try to free our large items for every 8 megabytes of additional large item
memory we allocate. This helps in particular on 32-bit builds and was noticable
in tst_qqmlecmascript::push_and_shift().

Change-Id: I4cc0b188e58ccaf32026e38c7aaf1cfadc83148b
Reviewed-by: Lars Knoll <lars.knoll@digia.com>