extlibs.rb: external libraries

* tool/extlibs.rb: download, extract, and patch external libraries.

* ext/fiddle/extlibs: libffi-3.2.1 and patch for mswin.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48935 b2dd03c8-39d4-4d8f-98ff-823fe69b080e