Rename A64 port to ARM64 port

BUG=354405
R=ulan@chromium.org, rodolph.perfetta@arm.com
LOG=y

Review URL: https://codereview.chromium.org/207823003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20148 ce2b1a6d-e550-0410-aec6-3dcde31c8c00