amd64: -m32 support for machine/pmap.h

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

Reviewed by:	jhb, imp

include/i386/Makefile

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

sys/amd64/include/pmap.h

@@ -44,6 +44,10 @@
  * $FreeBSD$
  */
 
+#ifdef __i386__
+#include <i386/pmap.h>
+#else /* !__i386__ */
+
 #ifndef _MACHINE_PMAP_H_
 #define	_MACHINE_PMAP_H_
 
@@ -584,3 +588,5 @@ pmap_pml5e_index(vm_offset_t va)
 #endif /* !LOCORE */
 
 #endif /* !_MACHINE_PMAP_H_ */
+
+#endif /* __i386__ */