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://golang.org/cl/136030043