cmd/internal/obj: convert Aconv to a stringer

cmd/internal/obj: convert Aconv to a stringer

Now that assembler opcodes have their own type, they can have a true
stringer, rather than explicit calls to Aconv, which makes for nicer
format strings.

Change-Id: Ic77f5f8ac38b4e519dcaa08c93e7b732226f7bfe
Reviewed-on: https://go-review.googlesource.com/25045
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>