[AIX] change "llvm-ar" to "env OBJECT_MODE=any llvm-ar" in clang/test for AIX OS

Summary:

In patch https://reviews.llvm.org/D127864. we add a new option -X for AIX OS, and default value is -X32. In order not effect the test cases in clang/test. we need to change "llvm-ar" to "env OBJECT_MODE=any llvm-ar" in clang/test for AIX OS

Differential Revision: https://reviews.llvm.org/D145600