cmd/go: don't always link in cgo for PIE

Internal linking for PIE is now supported and enabled by default
on some platforms, for which cgo is not needed. Don't always
bring in cgo.

Change-Id: I043ed436f0e6a3acbcc53ec543f06e193d614b36
Reviewed-on: https://go-review.googlesource.com/c/go/+/261498


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>