[clang] [Driver] Enable LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON on AIX. (PR #132821)

[Louis Dionne via cfe-commits]

https://github.com/ldionne commented:

I don't have comment on the diff but I think it's great that AIX is moving towards the mechanism that we want to use for all targets!

https://github.com/llvm/llvm-project/pull/132821