super: re-add set_cpu_flags.

It's used by ENV.fortran.

Fixes https://github.com/Homebrew/brew/issues/2530
Fixes https://github.com/Homebrew/homebrew-science/issues/5485

Library/Homebrew/extend/ENV/super.rb

@@ -325,6 +325,9 @@ module Superenv
 
   def set_x11_env_if_installed
   end
+
+  def set_cpu_flags(*)
+  end
 end
 
 class Array