[lit][AIX] Add LIBPATH to pass through env vars

AIX uses LIBPATH to specify the library search path in addition to
LD_LIBRARY_PATH, and a lot of users / tooling will use it
preferentially. In lit we currently pass through LD_LIBRARY_PATH but not
LIBPATH in the env on AIX, this patch corrects this inconsistency.

Differential Revision: https://reviews.llvm.org/D138510