Let function definitions in SMACK's libraries override these defined in input files

[shaobo-he]

Some real-world examples contain definitions of malloc, memcpy, memset, etc. If we just link them in, symbol conflicts arise. I think maybe we should set higher priorities for SMACK's definitions (i.e., let them override their counterparts in input files) via the --override flag of llvm-link.

[shaobo-he]

If we decide to do so, we have to make sure that our implementation of malloc returns before the actual body. I saw interesting bug like the custom malloc calls foo whereas foo I defined calls malloc.