Improve CodeHeap Free Space Management

The code in share/memory/heap.cpp could be improved by some small changes.

Furthermore, and in particular with these changes, the platform-specific parameter CodeCacheMinBlockLength should by fine-tuned to minimize the number of residual small free blocks.

Heap block allocation does not create free blocks smaller than CodeCacheMinBlockLength. This parameter value should match the minimal requested heap block size. If it is too small, such free blocks will never be re-allocated. They linger around at the beginning of the free list, slowing down the free block search.