On ia32, SAHF is guaranteed. It's only necessary to probe for it on x64.

R=svenpanne@chromium.org
BUG=376087
LOG=N

Review URL: https://codereview.chromium.org/296133006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21422 ce2b1a6d-e550-0410-aec6-3dcde31c8c00