Fix the following -Werror warning from clang 3.5.0, while building cddl/lib/libctf:

In file included from cddl/contrib/opensolaris/common/ctf/ctf_create.c:31:
In file included from sys/cddl/contrib/opensolaris/uts/common/sys/sysmacros.h:34:
sys/cddl/contrib/opensolaris/uts/common/sys/isa_defs.h:334:9: warning: '_ILP32' macro redefined [-Wmacro-redefined]
#define _ILP32

^

<built-in>:26:9: note: previous definition is here
#define _ILP32 1

^

1 warning generated.

This is because clang 3.5.0 started predefining _ILP32 and ILP32 for
the i386 arch. (Earlier versions already predefined _LP64 and LP64
for the x86_64 arch.)

Reviewed by: emaste, avg, smh, delphij, markj
Differential Revision: https://reviews.freebsd.org/D1187