mkmf.rb: default libdirname

* lib/mkmf.rb (MakeMakefile#init_mkmf): default libdirname to libdir.
* tool/rbinstall.rb: ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39488 b2dd03c8-39d4-4d8f-98ff-823fe69b080e