amd64: -m32 support for machine/pcpu.h

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

This is a kernel-only header and should not be required, but
procstat's zfs support includes this with _KERNEL defined.

Reviewed by:	jhb, imp

include/i386/Makefile

@@ -11,6 +11,10 @@ INCS=	\
 	profile.h \
 	segments.h \
 	vmparam.h
+# These kernel-only headers are used by procstat's ZFS support.
+# This should be fixed.
+INCS+=	\
+	pcpu.h
 INCSDIR=	${INCLUDEDIR}/i386
 
 .include <bsd.prog.mk>

sys/amd64/include/pcpu.h

@@ -28,6 +28,10 @@
  * $FreeBSD$
  */
 
+#ifdef __i386__
+#include <i386/pcpu.h>
+#else /* !__i386__ */
+
 #ifndef _MACHINE_PCPU_H_
 #define	_MACHINE_PCPU_H_
 
@@ -274,3 +278,5 @@ _Static_assert(sizeof(struct monitorbuf) == 128, "2x cache line");
 #endif /* _KERNEL */
 
 #endif /* !_MACHINE_PCPU_H_ */
+
+#endif /* __i386__ */