cmd/ld: diagnose Go calling C

For example:
go build -ldflags -C cmd/go 2>&1 | awk '{print $NF}' | sort | uniq -c | sort -nr

LGTM=khr
R=khr, josharian
CC=golang-codereviews
https://codereview.appspot.com/135170044
misc/cgo/testcdefs: comment out test of packed structs

The [568]c compilers no longer support packed structs, so
using them with -cdefs no longer works.  Just commenting out
the test, rather than removing it, in case this needs to be
handled.  It may be that -cdefs can go away entirely in the
future, in which case so can this directory.

LGTM=mdempsky
R=rsc, mdempsky
CC=golang-codereviews
https://codereview.appspot.com/136030043

Committer: Russ Cox <rsc@golang.org>