Use the pnacl-clang toolchain for (P)NaCL builds

Patch from JF Bastien <jfb@google.com>.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/631703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24402 ce2b1a6d-e550-0410-aec6-3dcde31c8c00