Fixes universal builds; Fixes Homebrew/homebrew#14845

Library/ENV/4.3/cc

@@ -120,12 +120,12 @@ class Cmd
     if cccfg? 'Ob'
       %w{-mtune=generic -Oz}
     elsif cccfg? 'O'
-      u = '-arch i386 -arch x86_64' if cccfg? 'u'
-      c = case tool when 'clang', 'clang++' then '-march=native' end
+      u = %w{-arch i386 -arch x86_64} if cccfg? 'u'
+      c = '-march=native' if tool =~ /clang/
       %w{-pipe -w -Os} << u << c
     else
       []
-    end
+    end.flatten
   end
   def syslibpath
     # We reject brew's lib as we explicitly add this as a -L flag, thus it