rlibc: fix bug in `memcmp()`
rlibc: add unit tests
auto merge of #15815 : errordeveloper/rust/rlibc_unit_tests, r=alexcrichton

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>