Use getauxval() if available.

This fixes the problem of not being able to detect ARM features on
ChromeOS because sandbox cannot access /proc/self/auxv and
/proc/cpuinfo there.

Drive-by-cleanup to libc detection in two other places.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/672543002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24789 ce2b1a6d-e550-0410-aec6-3dcde31c8c00