mkmf.rb: adjust default library path

* lib/mkmf.rb (MakeMakefile#init_mkmf): adjust default library path
  for multiarch.  [Bug #7874]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39371 b2dd03c8-39d4-4d8f-98ff-823fe69b080e