Disable red zones for amd64 FBSD kernel code

Disable red zones for amd64 FBSD kernel code

The kernel has been compiled with -mno-red-zone since at least
FreeBSD 5.0; when we compile libafs.ko with red zones enabled
and interact with the red-zone-less kernel, we get occasional
stack corruption, which manifests itself in a variety of
unpleasant ways.
The flags we pass to gcc for our module build currently differ
substantially from those used during a kernel build (or modules
built with the kernel); I hope to transition to a module build
process involving bsd.kmod.mk in the relatively near future.

Change-Id: I406e264c1f96e1d893b9d12b561f48a3cbfdf57c
Reviewed-on: http://gerrit.openafs.org/2297
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>