amd64: -m32 support for machine/cpufunc.h

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

Reviewed by:	jhb, imp

include/i386/Makefile

@@ -1,7 +1,8 @@
 # i386 headers installed on amd64
 
 .PATH: ${SRCTOP}/sys/i386/include
-INCS=
+INCS=	\
+	cpufunc.h
 INCSDIR=	${INCLUDEDIR}/i386
 
 .include <bsd.prog.mk>

sys/amd64/include/cpufunc.h

@@ -38,6 +38,10 @@
  * used in preference to this.
  */
 
+#ifdef __i386__
+#include <i386/cpufunc.h>
+#else /* !__i386__ */
+
 #ifndef _MACHINE_CPUFUNC_H_
 #define	_MACHINE_CPUFUNC_H_
 
@@ -966,3 +970,5 @@ int	wrmsr_safe(u_int msr, uint64_t newval);
 #endif
 
 #endif /* !_MACHINE_CPUFUNC_H_ */
+
+#endif /* __i386__ */