cmd/internal/objfile: provide consistent output in objdump on ppc64x

This makes a change to disasm.go so it provides consistent output
with the recent updates to arch/ppc64.

Change-Id: I812e5da2423fd1a84406032fd91ddf9cc86b7c69
Reviewed-on: https://go-review.googlesource.com/c/152761


Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>