[CMake][AIX] Fixing AIX rpath

Recent commit https://github.com/llvm/llvm-project/commit/8f833f88ab78265a8e0ebb0d1522771d67c708a9 modified the installation rpath and did not set `BUILD_WITH_INSTALL_RPATH` correctly on AIX, which led to installation failures on AIX. This patch sets `BUILD_WITH_INSTALL_RPATH` on AIX to fix the installation failures.

Reviewed By: buttaface, daltenty

Differential Revision: https://reviews.llvm.org/D148866