A64: Increase the margin for veneer checks.

BUG=v8:3168
LOG=N
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/171603008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19525 ce2b1a6d-e550-0410-aec6-3dcde31c8c00