Convert some ifdefs to use their V8_OS_* macros equivalents.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/127643002

Patch from Thiago Farina <tfarina@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18507 ce2b1a6d-e550-0410-aec6-3dcde31c8c00