build: Enable function sections and data sections during LTO.

This allows the linker to apply --gc-sections to object files created
during LTO. This yields a >8MB reduction in stripped binary size for
official binaries built with ThinLTO.

This change increases the size of an official binary built with regular
LTO by 8192 bytes, but this probably isn't worth worrying about.

BUG=660216,607968
R=thakis@chromium.org,krasin@chromium.org

Review-Url: https://codereview.chromium.org/2840723003
Cr-Commit-Position: refs/heads/master@{#467129}
1 file changed