amd64: -m32 support for machine/proc.h

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

Reviewed by:	jhb, imp

include/i386/Makefile

@@ -7,6 +7,7 @@ INCS=	\
 	atomic.h \
 	cpufunc.h \
 	pmap.h \
+	proc.h \
 	profile.h \
 	segments.h
 INCSDIR=	${INCLUDEDIR}/i386

sys/amd64/include/proc.h

@@ -32,6 +32,10 @@
  * $FreeBSD$
  */
 
+#ifdef __i386__
+#include <i386/proc.h>
+#else /* !__i386__ */
+
 #ifndef _MACHINE_PROC_H_
 #define	_MACHINE_PROC_H_
 
@@ -117,3 +121,5 @@ extern int max_ldt_segment;
 #endif  /* _KERNEL */
 
 #endif /* !_MACHINE_PROC_H_ */
+
+#endif /* __i386__ */