amd64: -m32 support for machine/atomic.h

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

Reviewed by:	jhb, imp

include/i386/Makefile

@@ -4,6 +4,7 @@
 INCS=	\
 	asm.h \
 	asmacros.h \
+	atomic.h \
 	cpufunc.h \
 	profile.h
 INCSDIR=	${INCLUDEDIR}/i386

sys/amd64/include/atomic.h

@@ -27,6 +27,11 @@
  *
  * $FreeBSD$
  */
+
+#ifdef __i386__
+#include <i386/atomic.h>
+#else /* !__i386__ */
+
 #ifndef _MACHINE_ATOMIC_H_
 #define	_MACHINE_ATOMIC_H_
 
@@ -592,3 +597,5 @@ atomic_swap_long(volatile u_long *p, u_long v)
 #endif /* !SAN_NEEDS_INTERCEPTORS || SAN_RUNTIME */
 
 #endif /* !_MACHINE_ATOMIC_H_ */
+
+#endif /* __i386__ */