Moving them up makes them properly aligned on 32-bit systems.
There are some odd fields above them right now
(like fixalloc and mutex maybe).

Change-Id: I57851a5bbb2e7cc339712f004f99bb6c0cce0ca5
Reviewed-on: https://go-review.googlesource.com/9889Reviewed-by: Austin Clements <austin@google.com>