shims/super/cc: tweak optimisation flag handling for runtime CPU detection builds

Let's reinstate adding our own optimisation flags if the compiler
was not invoked with a conflicting optimisation flag.

Library/Homebrew/shims/super/cc

@@ -308,7 +308,10 @@ class Cmd
 
     args << "-w" unless configure?
     args << "-#{ENV["HOMEBREW_OPTIMIZATION_LEVEL"]}"
-    args.concat(optflags) unless runtime_cpu_detection?
+    optflags.each do |optflag|
+      flag = optflag.split("=").first
+      args << optflag if !runtime_cpu_detection? || @args.none? { |arg| arg.start_with?(flag) }
+    end
     args.concat(archflags)
     args << "-std=#{@arg0}" if /c[89]9/.match?(@arg0)
     args << "-g" if debug_symbols?