This commit handles AIX cgo in cmd/nm tests.

Change-Id: I6753a0102e4f2c4c7bd4d7c999f62a0cb3d2183c
Reviewed-on: https://go-review.googlesource.com/c/go/+/164017
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>