Ensure that we use a 64-bit value for the last mmap() argument.

When using __syscall(2), the offset argument is passed on the stack on
amd64.  Previously only 32 bits were written, so the upper 32 bits were
garbage and could cause the test to fail.

MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

cddl/contrib/opensolaris/cmd/dtrace/test/tst/common/syscall/tst.args.c

@@ -35,7 +35,8 @@ int
 main(int argc, char **argv)
 {
 	for (;;) {
-		(void) __syscall(SYS_mmap, NULL, 1, 2, 3, -1, 0x12345678);
+		(void) __syscall(SYS_mmap, NULL, (size_t)1, 2, 3, -1,
+		    (off_t)0x12345678);
 	}
 
 	return (0);