ARM64: slightly modify weight for profiling counter

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

Review URL: https://codereview.chromium.org/252443004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20952 ce2b1a6d-e550-0410-aec6-3dcde31c8c00