Add unit test for rlibc

We are pulling in rlibc into Zinc (hackndev/zinc#113), but while reading the code, I though it could do with some unit tests.

<strike>So I have added some tests here, but seem like there are a few issues... Please see comments with `XXX` and respond if you have any thoughts...</strike>
Hm, just thinking about it: you could just import these names to avoid the `::`. `use super::{memcmp, memset, memcpy, memmove};`.