amd64: -m32 support for machine/segments.h

Install the i386 segments.h under /usr/include/i386 on amd64 and include
when targeting i386.

Reviewed by:	jhb, imp

include/i386/Makefile

@@ -6,7 +6,8 @@ INCS=	\
 	asmacros.h \
 	atomic.h \
 	cpufunc.h \
-	profile.h
+	profile.h \
+	segments.h
 INCSDIR=	${INCLUDEDIR}/i386
 
 .include <bsd.prog.mk>

sys/amd64/include/segments.h

@@ -36,6 +36,10 @@
  * $FreeBSD$
  */
 
+#ifdef __i386__
+#include <i386/segments.h>
+#else /* !__i386__ */
+
 #ifndef _MACHINE_SEGMENTS_H_
 #define	_MACHINE_SEGMENTS_H_
 
@@ -105,3 +109,5 @@ void	update_gdt_fsbase(struct thread *td, uint32_t base);
 #endif /* _KERNEL */
 
 #endif /* !_MACHINE_SEGMENTS_H_ */
+
+#endif /* __i386__ */