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

sys/cddl/contrib/opensolaris/uts/common/sys/isa_defs.h

@@ -331,7 +331,9 @@ extern "C" {
 /*
  * Define the appropriate "implementation choices".
  */
+#if !defined(_ILP32)
 #define	_ILP32
+#endif
 #if !defined(_I32LPx) && defined(_KERNEL)
 #define	_I32LPx
 #endif