vm_method.c: disable GMC writing if GMC is disabled

* vm_method.c (rb_method_entry_get_without_cache): disable GMC
  writing if GMC is disabled
  [ruby-core:61218]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45261 b2dd03c8-39d4-4d8f-98ff-823fe69b080e