[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:

> > Actually, `compiler-rt` build on AIX doesn't call `get_compiler_rt_target`, so it shouldn't be affected.
> 
> How so? What is stopping someone to set `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON` on AIX? Is it just not an unsupported configuration?

Actually, you are right. We do call `get_compiler_rt_target` when `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON`.

I think the comment in `flang-rt/cmake/modules/GetToolchainDirs.cmake` may be incorrectly flipped. It has
```
# * LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=1: lib/${oslibname}/libclang_rt.builtins-${arch}.a
# * LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=0: lib/${triple}/libclang_rt.builtins.a
```
However, on AIX,

When it is ON, we have `lib/clang/21/lib/powerpc-ibm-aix/libclang_rt.builtins.a`
When it is OFF, we have `lib/clang/21/lib/aix/libclang_rt.builtins-powerpc64.a`

which is exactly the opposite of what the flang-rt comment says.

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