cmd/internal/obj: unify Setuintxx and WriteInt

cmd/internal/obj: unify Setuintxx and WriteInt

They do basically the same work.

Setuintxx was only used in a single place,
so eliminate it in favor of WriteInt.

duintxxLSym's alignment rounding was not used in practice;
change it into alignment assertion.

Passes toolstash-check. No compiler performance changes.

Change-Id: I0f7410cf2ccffbdc02ad796eaf973ee6a83074f8
Reviewed-on: https://go-review.googlesource.com/40863
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>