GN: match shell behavior for cc and cxx in is_clang.py

Interprets cc/cxx as part of the shell command rather than literal
arguments. This matches the behavior of shell invocations from ninja.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2375573002

Review-Url: https://codereview.chromium.org/2375573002

gn/is_clang.py

@@ -9,8 +9,8 @@ import subprocess
 import sys
 cc,cxx = sys.argv[1:3]
 
-if ('clang' in subprocess.check_output([cc,  '--version']) and
-    'clang' in subprocess.check_output([cxx, '--version'])):
+if ('clang' in subprocess.check_output('%s --version' % cc, shell=True) and
+    'clang' in subprocess.check_output('%s --version' % cxx, shell=True)):
   print 'true'
 else:
   print 'false'