Now that there's no 6c compiler anymore, there's no need for cgo to
generate C headers that are compatible with it.

Fixes #9528

Change-Id: I43f53869719eb9a6065f1b39f66f060e604cbee0
Reviewed-on: https://go-review.googlesource.com/2482Reviewed-by: Ian Lance Taylor <iant@golang.org>