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>
This should be removed: https://github.com/rust-lang/rust/pull/15815#discussion_r15147905