Fix gcc vs clang cpuid.h discrepency.

For r1254, gcc defines bit_FXSAVE while clang defines bit_FXSR.

R=mark@chromium.org

Review URL: https://breakpad.appspot.com/924002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1260 4c0a9323-5329-0410-9bdc-e9ce6186880e