[RFC] Adding 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.

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 :+1:
However I don't think we want to be testing the exact value? Does the C standard require that the returned value is exactly this difference ([the man page](http://man.cx/memcmp) just talks about the sign). I.e. I think it may be better for this test to be `assert!(::memcmp(...) > 0)`, and below, `assert!(::memcmp(...) < 0)`.