cmd/objdump: disable external linking test on openbsd/arm

Disable disassembly with external linking test on openbsd/arm, since this
platform does not currently support cgo/external linking.

Change-Id: I6eab6fcaac21407ce05075a4a1407fbfe0e6142b
Reviewed-on: https://go-review.googlesource.com/9481


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>