Currently _MaxMem is a uintptr, which is going to complicate some
further changes. Make it untyped so we'll be able to do untyped math
on it before truncating it to a uintptr.

The runtime assembly is identical before and after this change on
{linux,windows}/{amd64,386}.

Updates #18651.

Change-Id: I0f64511faa9e0aa25179a556ab9f185ebf8c9cf8
Reviewed-on: https://go-review.googlesource.com/35251
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>