amd64: -m32 support for machine/counter.h

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

This is a kernel-only header required by procstat's ZFS support.

Reviewed by:	jhb, imp

include/i386/Makefile

@@ -14,6 +14,7 @@ INCS=	\
 # These kernel-only headers are used by procstat's ZFS support.
 # This should be fixed.
 INCS+=	\
+	counter.h \
 	pcpu.h \
 	pcpu_aux.h
 INCSDIR=	${INCLUDEDIR}/i386

sys/amd64/include/counter.h

@@ -28,6 +28,10 @@
  * $FreeBSD$
  */
 
+#ifdef __i386__
+#include <i386/counter.h>
+#else /* !__i386__ */
+
 #ifndef __MACHINE_COUNTER_H__
 #define __MACHINE_COUNTER_H__
 
@@ -90,3 +94,5 @@ counter_u64_add(counter_u64_t c, int64_t inc)
 }
 
 #endif	/* ! __MACHINE_COUNTER_H__ */
+
+#endif /* __i386__ */