Add a script to delete stale zero-byte build files
https://bugs.webkit.org/show_bug.cgi?id=106131

Reviewed by Tony Chang.

Delete zero-byte build files on Mac port for now. It doesn't do anything on other ports.

* BuildSlaveSupport/build.webkit.org-config/master.cfg:
(DeleteStaleBuildFiles): Added.
(Factory.__init__): Add DeleteStaleBuildFiles step.
* BuildSlaveSupport/delete-stale-build-files: Added.
(main): Delete zero-byte files except ones intentionally generated by our build process.
(webkitBuildDirectory): Copied from build-product-archive.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138888 268f45cc-cd09-0410-ab3c-d52691b4dbfc