Consistently use int16 for [ft]reg and int64 for [ft]offset.

Change-Id: I7d279bb6e4fb735105429234a949074bf1cefb29
Reviewed-on: https://go-review.googlesource.com/29215
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>