[llvm] [flang-rt] replace the triple dir to 'aix' for flang-rt to be consistent with clang on AIX. (PR #130875)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

> You are correct, fixed in [13261e8](https://github.com/llvm/llvm-project/commit/13261e856aa54e6c86b500c6d0bb329f08ad5eb7).
> 
> As the comment mentioned, flang-rt only uses the LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON scheme and ignores the setting. See [#110217 (comment)](https://github.com/llvm/llvm-project/pull/110217#issuecomment-2557126247) for the decision.
> 
> I am LGTM-ing this low-impact PR in order to get it working on AIX even though I think there is something more profound going on. It can be cleaned up some other time.

Thanks for the review!
We are not planning to support `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON` on AIX for both `flang` and `clang`, so this PR is setting the library path for `flang-rt` as if the option is OFF.
I am planning to also change the library name to include `${arch}` to complete the reverse. 
I am not sure if you plan to change the default of the option in the future, but we can deal with that if it is the case.

https://github.com/llvm/llvm-project/pull/130875