[lit] Set shlibpath_var on AIX
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
Differential Revision: https://reviews.llvm.org/D59741