cleaner: fix generic executable handling. (#498)

Don't unset things that are already set as executable for the generic path.
OS X overrides this behaviour and detects if it's an MachO executable but
there's no cross-OS generic equivalent so just fall back to whatever the build
system has set.

Library/Homebrew/cleaner.rb

@@ -67,7 +67,7 @@ class Cleaner
   end
 
   def executable_path?(path)
-    path.text_executable?
+    path.text_executable? || path.executable?
   end
 
   # Clean a top-level (bin, sbin, lib) directory, recursively, by fixing file