Use RBasic padding for embedded string on 64-bit CPU

On 64-bit CPU, there is padding in `RBasic`, so reorder the fields and use
it as buffer of embedded string. This change allows 4 more bytes to be
embedded on 64-bit CPU.

However, an incompatibility will occur if `RString::as::ary` is accessed
directly because `RString` structure has changed.

include/mruby/object.h

@@ -8,11 +8,11 @@
 #define MRUBY_OBJECT_H
 
 #define MRB_OBJECT_HEADER \
+  struct RClass *c;       \
+  struct RBasic *gcnext;  \
   enum mrb_vtype tt:8;    \
   uint32_t color:3;       \
-  uint32_t flags:21;      \
-  struct RClass *c;       \
-  struct RBasic *gcnext
+  uint32_t flags:21
 
 #define MRB_FLAG_TEST(obj, flag) ((obj)->flags & (flag))