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

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

Committed: https://code.google.com/p/v8/source/detail?r=21901

[Jakob Kummerow, 2014-06-20 12:22:23]

This is per https://codereview.chromium.org/339343002/#msg7. 

While I was at it, I cleaned up the gcc version as well.

[Alexander Potapenko, 2014-06-20 12:25:21]

LGTM, thanks!

[Sven Panne, 2014-06-20 12:29:00]

LGTM, too, but can we please add the relevant Chromium commit numbers to this
CL? If I see it correctly, we simply track a handful of them here, and that's
not clear initially.

[Jakob Kummerow, 2014-06-20 12:59:02]

Committed patchset #1 manually as r21901 (presubmit successful).