ensure host compiler is always clang when cross compiling from linux

BUG=

Review URL: https://codereview.chromium.org/955393002

Cr-Commit-Position: refs/heads/master@{#26881}

build/standalone.gypi

@@ -86,6 +86,7 @@
     'werror%': '-Werror',
     'use_goma%': '<(use_goma)',
     'gomadir%': '<(gomadir)',
+    'host_clang%': '1',
 
     # .gyp files or targets should set v8_code to 1 if they build V8 specific
     # code, as opposed to external code.  This variable is used to control such
@@ -333,6 +334,26 @@
           }],
         ],
       },
+      'conditions': [
+        ['clang!=1 and host_clang==1 and target_arch!="ia32" and target_arch!="x64"', {
+          'make_global_settings': [
+            ['CC.host', '<(clang_dir)/bin/clang'],
+            ['CXX.host', '<(clang_dir)/bin/clang++'],
+          ],
+        }],
+        ['clang==0 and host_clang==1', {
+          'target_conditions': [
+            ['_toolset=="host"', {
+              'cflags_cc': [ '-std=gnu++11', ],
+            }],
+          ],
+          'target_defaults': {
+            'target_conditions': [
+              ['_toolset=="host"', { 'cflags!': [ '-Wno-unused-local-typedefs' ]}],
+            ],
+          },
+        }],
+      ],
     }],
     # 'OS=="linux" or OS=="freebsd" or OS=="openbsd" or OS=="solaris"
     #  or OS=="netbsd"'