common.mk: update-gems for older BASERUBY

* common.mk (update-gems): chdir to the target directory and then
  add the tool directory to load paths, for older BASERUBY.
  [Bug #10372][ruby-core:65630]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47904 b2dd03c8-39d4-4d8f-98ff-823fe69b080e