ARM64: Generate optimized code for Math.floor and Math.round with double outputs.

R=jkummerow@chromium.org, ulan@chromium.org

Review URL: https://codereview.chromium.org/258793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21091 ce2b1a6d-e550-0410-aec6-3dcde31c8c00