mkmf.rb: fail if cross compiling

* lib/mkmf.rb (MakeMakefile#try_run): bail out explicitly if cross
  compiling, because it cannot work of course.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39252 b2dd03c8-39d4-4d8f-98ff-823fe69b080e