Revert of Reland Make clang the default on linux. (patchset #3 id:40001 of https...

Revert of Reland Make clang the default on linux. (patchset #3 id:40001 of https://codereview.chromium.org/861223002/)

Reason for revert:
Still missing these builders:
http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20arm%20-%20sim
http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20arm%20-%20sim%20-%20debug
http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20full%20debug

Original issue's description:
> Reland Make clang the default on linux.
>
> BUG=
>
> Committed: https://crrev.com/72ef6843b0aff68e519a8f26e7d2f7dd87b4712c
> Cr-Commit-Position: refs/heads/master@{#26204}

TBR=jochen@chromium.org,jkummerow@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/864093004

Cr-Commit-Position: refs/heads/master@{#26205}