Reland Make clang the default on linux.

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

Review URL: https://codereview.chromium.org/862173002

Cr-Commit-Position: refs/heads/master@{#26206}

build/standalone.gypi

@@ -146,7 +146,7 @@
       }, {
         'v8_enable_gdbjit%': 0,
       }],
-      ['OS=="mac"', {
+      ['(OS=="linux" or OS=="mac") and (target_arch=="ia32" or target_arch=="x64")', {
         'clang%': 1,
       }, {
         'clang%': 0,