Fix compatibility issue of class variables.

Singleton class definition do not introduce its own class variable scope
in CRuby/JRuby. So should mruby.

```
module Mod1
  class << Object.new

    C = 1
    @@cv = 1

    p Module.nesting,        # => [#<Class:#<Object:0x55cb16e60a50>>, Mod1]
      constants,             # => [:C]
      class_variables,       # => []
      Mod1.class_variables   # => [:@@cv]
  end
end
```

src/variable.c

@@ -743,7 +743,13 @@ mrb_vm_cv_get(mrb_state *mrb, mrb_sym sym)
 {
   struct RClass *c;
 
-  c = MRB_PROC_TARGET_CLASS(mrb->c->ci->proc);
+  struct RProc *p = mrb->c->ci->proc;
+
+  for (;;) {
+    c = MRB_PROC_TARGET_CLASS(p);
+    if (c->tt != MRB_TT_SCLASS) break;
+    p = p->upper;
+  }
   return mrb_mod_cv_get(mrb, c, sym);
 }
 
@@ -751,8 +757,13 @@ void
 mrb_vm_cv_set(mrb_state *mrb, mrb_sym sym, mrb_value v)
 {
   struct RClass *c;
+  struct RProc *p = mrb->c->ci->proc;
 
-  c = MRB_PROC_TARGET_CLASS(mrb->c->ci->proc);
+  for (;;) {
+    c = MRB_PROC_TARGET_CLASS(p);
+    if (c->tt != MRB_TT_SCLASS) break;
+    p = p->upper;
+  }
   mrb_mod_cv_set(mrb, c, sym, v);
 }