A char array copy optimization (libcore).

- 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

(cherry picked from commit a3eb10feba1c3479ad99ef5c5323d011a5365f4b)

Change-Id: Ic002ced3bc2f2a682cd4b8cec07ede990edd7463