Don't register non-heap allocated objects

`rb_define_const` can add objects as "mark objects".  This is to make
code like this work:

  https://github.com/ruby/ruby/blob/33d6e92e0c6eaf1308ce7108e653c53bb5fb106c/ext/etc/etc.c#L1201

```
    rb_define_const(rb_cStruct, "Passwd", sPasswd); /* deprecated name */
```

sPasswd is a heap allocated object that is also a C global, so we can't
move it (it needs to be pinned).  However, we have many calls to
`rb_define_const` that just pass in an integer like this:

```
rb_define_const(rb_cDBM, "WRITER",  INT2FIX(O_RDWR|RUBY_DBM_RW_BIT));
```

Non heap allocated objects like integers will never move, so there is no
reason to waste time in the GC marking / pinning them.

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