A char array copy optimization (art).

- Based on measurements, copy char by char for arrays of length <= 64.
- With this change, the Ritz MemAllocBench got ~25% faster on Nexus 4
  and ~20% faster on host.
- This change only handles arraycopy calls in the core libraries and
  char arrays with the rest future work.

Bug: 7103825

Change-Id: If536f9ea68ecf277b604199130d2060c446f640c