ARM: Small copy optimization. Copying 64bits at a time.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/14121006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14254 ce2b1a6d-e550-0410-aec6-3dcde31c8c00