proc.c: frozen core methods

* proc.c (mproc, mlambda): use frozen core methods instead of plain
  global methods, so that methods cannot be overridden.
  [ruby-core:54687] [Bug #8345]
* vm.c (Init_VM): define proc and lambda on the frozen core object.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40528 b2dd03c8-39d4-4d8f-98ff-823fe69b080e