cmd/objdump: add support for -gnu option on Go objdump

This adds support for the -gnu option on Go objdump. When
this option is used, then output will include gnu
assembly in comments alongside the Go assembly.

The objdump test was updated to test this new option.

This option is supported for the arches found in
golang.org/x that provide the GNUsyntax function.

Updates #34372

Change-Id: I9e60e1691526607dda3c857c4564dcef408b8391
Reviewed-on: https://go-review.googlesource.com/c/go/+/225459


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>