extend/ENV/super: add ENV.O3

Having this would have been, or would be, useful on multiple occassions:

- Homebrew/homebrew-core#94724
- Homebrew/homebrew-core#136551
- #15372

I think it's time that we added this back.

Library/Homebrew/extend/ENV/super.rb

@@ -375,6 +375,15 @@ module Superenv
       self["HOMEBREW_OPTIMIZATION_LEVEL"] = "O1"
     end
   end
+
+  sig { params(block: T.nilable(T.proc.void)).void }
+  def O3(&block)
+    if block
+      with_env(HOMEBREW_OPTIMIZATION_LEVEL: "O3", &block)
+    else
+      self["HOMEBREW_OPTIMIZATION_LEVEL"] = "O3"
+    end
+  end
   # rubocop: enable Naming/MethodName
 end