ARM64: Use templates for simulated memory accesses.

This cleans up the memory interface somewhat, and also avoids a memcpy
call that the compiler can't optimise away.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/536923002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23640 ce2b1a6d-e550-0410-aec6-3dcde31c8c00