RTS configure: Move over mem management checks

These are for heap allocation, a strictly RTS concern.

All of this should boil down to `AC_DEFINE` not `AC_SUBST`, so it
belongs in the RTS configure and should be safe to move without
modification.

The RTS configure one has a new
```
AC_CHECK_SIZEOF([void *])
```
that the top-level configure version didn't have, so that
`ac_cv_sizeof_void_p` is defined. Once more code is moved over in latter
commits, that can go away.

Progress towards #17191