doc: update /doc/asm compiler output example

doc: update /doc/asm compiler output example

The compiler output shown in the doc is now quite old
(most of the changes happened in Go 1.5).
Update it to be more like what users will actually see.

Also explain how to get literal machine code again.

Prompted by #30968.

Change-Id: I0ce139c3fe299ccc43e85b6aca81c6e0aac1a2df
Reviewed-on: https://go-review.googlesource.com/c/go/+/175757
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>