linux: enable safe identical code folding for release builds
linux: enable safe identical code folding for release builds

This shaves 2.5mb (~5%) off the binary, at the cost of ~1 second
(~25%) of linking.  In debug builds, the 25% time cost is much
larger so we only enable this for Release mode.

BUG=76280

Review URL: http://codereview.chromium.org/6931008

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@84088 0039d316-1c4b-4281-b951-d872f2087c98

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