[clang] [llvm] [flang-rt] Pass the whole path of libflang_rt.runtime.a to linker on AIX (PR #131041)

[Daniel Chen via llvm-commits]

DanielCChen wrote:

@Meinersbur @MaskRay and all other reviewers,

PR #132821 enables `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON` on AIX. As a result, I no longer need  to "force" AIX to use `os_dirname` for flang-rt. 

The change in this PR is intact because I re-used the same code to handle the option either ON or OFF.

I updated the description of this PR to reflect that.

I will undo PR #130875 and add a LIT test to this PR as soon as PR #132821 lands. 

Sorry about the change or direction.

https://github.com/llvm/llvm-project/pull/131041