libc - Revert to nmalloc for now

* Disable dmalloc on 64-bit systems, use nmalloc for everything for now.
  dmalloc appears to have some severe fragmentation and/or leak issues
  in long running programs (aka xulrunner) which are worse than nmalloc.
libc - Add poor man's cache coloring optimization to nmalloc module.

* A series of large allocations in excess of 32KB will be offset by 4K from
  each other.  This fixes performance issues on SandyBridge and later cpus
  related to large matrix operations.

  This eats an extra 4K of VM for such allocations but does not eat any
  additional real memory.

* Greatly improves large FP matrix benchmarks.  Real-world effects are more
  questionable.

* The Sandybridge and later cpus use a virtually indexed, physically tagged
  L1 cache, and tend to be sensitive to substantially different memory
  addresses winding up on the same cache line.  Matrix operations (primarily
  benchmarks) can cause these sorts of effects.

Reported-by: alexh