[AIX][clang][driver] Restrict /usr/lib to internal library search paths

Adding it to the general filepaths results in it being added to the
linker arguments. The AIX linker always looks in this path anyway
and adds it as a default library path component. Adding this duplicate
explicitly results in duplicate entries in path in the loader section
of executables and messes up tools like CMake that parse the default
library flags.

Reviewed By: ZarkoCA

Differential Revision: https://reviews.llvm.org/D97574