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

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>