Backtrace for SystemStackError

* eval.c (setup_exception): set backtrace in system stack error
  other than the pre-allocated sysstack_error.  [Feature #6216]
* proc.c (Init_Proc): freeze the pre-allocated sysstack_error.
* vm_insnhelper.c (vm_stackoverflow): raise new instance for each
  times without calling any methods to keep the backtrace with no
  further stack overflow.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46502 b2dd03c8-39d4-4d8f-98ff-823fe69b080e