Fix dumping of io syscalls when descriptor argument has unused bits set

Fix dumping of io syscalls when descriptor argument has unused bits set

* syscall.c (dumpio): Explicitly cast the first argument of syscall
to "int", the same way as the kernel does.
* tests/read-write.c: Include <asm/unistd.h> and "kernel_types.h".
(k_read, k_write): New functions.
(test_dump, main): Use them.