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

[Daniel Chen via cfe-commits]

DanielCChen wrote:

Thanks to @MaskRay for reviewing the PR and @ldionne for the comments that led us to find another issue. 
Would the reviewers please let me know if you are still reviewing the change? I am planning to merge it if you have no further comments.

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