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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Not planning to implement support for honoring `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` myself. I think the idea of LLVM move away from LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF as a configuration option, it just adds complexity/differences. The resource directory is also private to Clang, its organization should not be a concern of the host platform. However, I would not object to someone implementing it.

Consider that there already an exception for Apple:
https://github.com/llvm/llvm-project/blob/bbd1bb40573dd5a36dc04af4ad307d6e16456f89/flang-rt/cmake/modules/GetToolchainDirs.cmake#L37
Consider using `os_dirname` instead of modigying `get_toolchain_arch_dirname`.

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