configure.in: no runtime path to -L

* configure.in (LIBRUBY_RPATHFLAGS): do not append -L option with
  runtime library directory if cross compiling, but only -R option.
  runtime path makes no sense on the host system.  [ruby-dev:47363]
  [Bug #8443]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40907 b2dd03c8-39d4-4d8f-98ff-823fe69b080e