[llvm] [AIX] use LIBPATH on AIX instead of LD_LIBRARY_PATH (PR #94602)

[Anh Tuyen Tran via llvm-commits]

================
@@ -588,7 +588,10 @@ def use_clang(
             if getattr(self.config, pp, None)
         ]
 
-        self.with_environment("LD_LIBRARY_PATH", lib_paths, append_path=True)
+        if sys.platform.startswith("aix"):
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anhtuyenibm wrote:

I agreed with RoboTux @RoboTux, can we try something like line 106
```
        elif platform.system() == "AIX":
            features.add("system-aix") 
```

https://github.com/llvm/llvm-project/pull/94602