cmd/objdump: skip PIE test when cgo is disabled and PIE needs external linking

On some platforms, building PIE needs external linking, which
cannot run if cgo is not available.

Change-Id: I6d504aed0f0442cda0355d0beac606ad365e2046
Reviewed-on: https://go-review.googlesource.com/c/go/+/447616


Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>