Revert of Make clang the default on linux. (patchset #2 id:20001 of https://codereview.chromium.org/781983002/)

Reason for revert:
breaks arm and mips compilation on main waterfall

Original issue's description:
> Make clang the default on linux.
>
> TBR=jochen@chromium.org
>
> Committed: https://crrev.com/006b5c5cfd8517563601f331bc7a57e13bfeb202
> Cr-Commit-Position: refs/heads/master@{#26164}

TBR=machenbach@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/861553004

Cr-Commit-Position: refs/heads/master@{#26166}