Use new specifiers/modifiers of `mrb_vfromat()`

The binary sizes (gems are only `mruby-bin-mruby`) are reduced slightly in
my environment than before the introduction of new specifiers/modifiers
(5116789a) with this change.

  ------------+-------------------+-------------------+--------
   BINARY     | BEFORE (5116789a) |   AFTER (This PR) |  RATIO
  ------------+-------------------+-------------------+--------
   mruby      |      593416 bytes |      593208 bytes | -0.04%
   libmruby.a |      769048 bytes |      767264 bytes | -0.23%
  ------------+-------------------+-------------------+--------

BTW, I accidentally changed `tasks/toolchains/visualcpp.rake` at #4613,
so I put it back.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/gc.c b/src/gc.c
index b05d929a1..a7a67ebfa 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -542,7 +542,7 @@ mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
         ttype != MRB_TT_ICLASS &&
         ttype != MRB_TT_ENV &&
         ttype != tt) {
-      mrb_raisef(mrb, E_TYPE_ERROR, "allocation failure of %S", mrb_obj_value(cls));
+      mrb_raisef(mrb, E_TYPE_ERROR, "allocation failure of %C", cls);
     }
   }