Prepare switch from a64 to arm64

BUG=354405
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/207793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20147 ce2b1a6d-e550-0410-aec6-3dcde31c8c00