amd64: -m32 support for machine/sb_buf.h

The contents of the amd64 version are kernel-only and incompatible with
other headers when compiled for i386 userspace with _KERNEL defined.
Just ifdef the whole file out in that case rather than giving this file
the full x86 treatment since it's not needed for current use cases
(procstat zfs support).

Reviewed by: jhb, imp