Enable ThinLTO for POSIX LTO by default on Linux.

This is a reland of the remaining part of
https://codereview.chromium.org/2831213006 .

The binary size regression has been addressed by disabling ThinLTO
whole-program optimizations and improving the quality of unoptimized
code.

BUG=660216
R=thakis@chromium.org,hans@chromium.org

Review-Url: https://codereview.chromium.org/2939373002
Cr-Commit-Position: refs/heads/master@{#480144}
1 file changed