sahf flag will not be set for ia32 on some old platform because some old processors does not support CPUID's extended features.
This also avoids redundant cpuid check in ia32 for sahf.

BUG=
R=danno@chromium.org

Review URL: https://codereview.chromium.org/298823008

Patch from Weiliang Lin <weiliang.lin@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21766 ce2b1a6d-e550-0410-aec6-3dcde31c8c00