Patch from bradchen@chromium.org.

Prevents use of PROT_EXEC for NaCl builds

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/19729003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15744 ce2b1a6d-e550-0410-aec6-3dcde31c8c00