Work around cp breakage in current from last week

There was a small window cp was broken. Work around this by using :>
instead of cp /dev/null. Ideally, we'd keep the cp /dev/null in the
build as a regression test, but doing so breaks people that upgraded
during the cp breakage and this is simpler than bootstrapping a
working cp since there's no good __FreeBSD_version sign posts for
that.

Suggested by: lots of people
Too stubborn for his own good: imp

stand/i386/zfsboot/Makefile

@@ -81,7 +81,7 @@ zfsboot.ld: zfsboot.ldr zfsboot.bin ${BTXKERN}
 	    -o ${.TARGET} -P 1 zfsboot.bin
 
 zfsboot.ldr:
-	cp /dev/null ${.TARGET}
+	:> ${.TARGET}
 
 zfsboot.bin: zfsboot.out
 	${OBJCOPY} -S -O binary zfsboot.out ${.TARGET}

stand/libsa/Makefile

@@ -122,7 +122,7 @@ beforedepend:
 	ln -sf ${SRCTOP}/include/arpa/inet.h arpa/inet.h; \
 	ln -sf ${SRCTOP}/include/arpa/tftp.h arpa/tftp.h; \
 	for i in _time.h _strings.h _string.h; do \
-		[ -f xlocale/$$i ] || cp /dev/null xlocale/$$i; \
+		[ -f xlocale/$$i ] || :> xlocale/$$i; \
 	done; \
 	for i in ${STAND_H_INC}; do \
 		ln -sf ${SASRC}/stand.h $$i; \