ARM64: Change some incorect uses of ptrdiff_t to int64_t.

R=bmeurer@chromium.org, ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/559073003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23856 ce2b1a6d-e550-0410-aec6-3dcde31c8c00