load.c: display backtrace to $stderr

* load.c (load_lock): display backtrace to $stderr at circular
  require.
* vm_backtrace.c (rb_backtrace_print_to): new function to print
  backtrace to the given output.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43204 b2dd03c8-39d4-4d8f-98ff-823fe69b080e