Fix compilation on x64 architectures

Contributed by Mathias Bynens <mathiasb@opera.com>.

BUG=v8:3110
LOG=Y
R=bmeurer@chromium.org, jarin@chromium.org

Review URL: https://codereview.chromium.org/143003007

Patch from Mathias Bynens <mathiasb@opera.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18760 ce2b1a6d-e550-0410-aec6-3dcde31c8c00