encoding/binary: check for unsigned integers in intDataSize.

intDataSize ignores unsigned integers, forcing reads/writes to miss the fast path.

Fixes #8956

Change-Id: Ie79b565b037db3c469aa1dc6d0a8a5a9252d5f0a
Reviewed-on: https://go-review.googlesource.com/1777
Reviewed-by: Russ Cox <rsc@golang.org>