cmd/link: disable plugin support if cgo is disabled

Functional plugin support requires cgo to be enabled. Disable
it if the environment has disabled cgo.

This prevents unexpected linker failures when linking large
binaries with cgo disabled which use the plugin package.

Fixes #45564

Change-Id: Ib71f0e089f7373b7b3e3cd53da3612291e7bc473
Reviewed-on: https://go-review.googlesource.com/c/go/+/314449


Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>