common.mk: uninstall

* common.mk (uninstall): uninstall installed file listed in
  $(INSTALLED_LIST) file.

* tool/rbuninstall.rb: simple uninstallation tool.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39339 b2dd03c8-39d4-4d8f-98ff-823fe69b080e