mkconfig.rb: fix empty prefix

* tool/mkconfig.rb: fix empty RbConfig::CONFIG["prefix"] when
  configured --with-rubyarchprefix, remove prefix from rubyarchdir
  after expansion for the case it does not start with '$(prefix)'.
  [fix GH-643]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46586 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-06-28

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46587 b2dd03c8-39d4-4d8f-98ff-823fe69b080e