(*sigctxt).fault() currently returns either uintptr, uint32, or uint64
depending on the platform. Make them all return uintptr.

For #10958 (but a nice change on its own).

Change-Id: I7813e779d0edcba112dd47fda776f4ce6e50e227
Reviewed-on: https://go-review.googlesource.com/68015
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>