amd64: -m32 support for machine/md_var.h

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

This is a mostly kernel-only header required by procstat's ZFS support.
It is pulled in by the i386 machine/counter.h.

Reviewed by:	jhb, imp

include/i386/Makefile

@@ -15,6 +15,7 @@ INCS=	\
 # This should be fixed.
 INCS+=	\
 	counter.h \
+	md_var.h \
 	pcpu.h \
 	pcpu_aux.h
 INCSDIR=	${INCLUDEDIR}/i386

sys/amd64/include/md_var.h

@@ -31,6 +31,10 @@
  * $FreeBSD$
  */
 
+#ifdef __i386__
+#include <i386/md_var.h>
+#else /* !__i386__ */
+
 #ifndef _MACHINE_MD_VAR_H_
 #define	_MACHINE_MD_VAR_H_
 
@@ -95,3 +99,5 @@ int	set_fpcontext(struct thread *td, struct __mcontext *mcp,
 	    char *xfpustate, size_t xfpustate_len);
 
 #endif /* !_MACHINE_MD_VAR_H_ */
+
+#endif /* __i386__ */