[lit] Make the gold plugin support testing work with a python3

interpreter. Seems that's a better path than pinning to python2.7.

Thanks to Justin for prodding me toward a fix. =]

llvm-svn: 229247

llvm/test/lit.cfg

@@ -353,7 +353,7 @@ def have_ld_plugin_support():
         return False
 
     ld_version = subprocess.Popen(['ld', '--version'], stdout = subprocess.PIPE)
-    if not 'GNU gold' in ld_version.stdout.read():
+    if not 'GNU gold' in ld_version.stdout.read().decode():
         return False
     ld_version.wait()