mkconfig.rb: clear exec_prefix

* tool/mkconfig.rb: clear exec_prefix which may differ from prefix,
  before expanding rubyarchdir to remove prefix.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39297 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
configure.in: unexpand exec_prefix

* configure.in (exec_prefix): unexpand after RUBY_EXEC_PREFIX is set.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39298 b2dd03c8-39d4-4d8f-98ff-823fe69b080e