Make heap numbers detection more consistent on arm64.
All the tested benchmarks (octane2, kraken, sunspider, v8-v4 and lua) are unchanged (a57 and a53).

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

Review URL: https://codereview.chromium.org/577273002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24176 ce2b1a6d-e550-0410-aec6-3dcde31c8c00