[lit] Set shlibpath_var on AIX

Summary:
When building the `check-all` target on AIX, lit produces
```
warning: unable to inject shared library path on 'AIX'
```

This patch addresses this. `LIBPATH` is the environment variable of
interest on AIX. Newer versions of AIX may consider `LD_LIBRARY_PATH`,
but only when `LIBPATH` is unset.

Reviewers: xingxue, jasonliu, sfertile, serge-sans-paille

Reviewed By: xingxue

Subscribers: jsji, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59741

llvm-svn: 357334