Clean up unused stuff in atomicops_internals_{tsan,x86_gcc}.h

This ports crrev.com/278081 and crrev.com/271506 to V8.

R=glider@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/342323002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21901 ce2b1a6d-e550-0410-aec6-3dcde31c8c00