[clang] XFAIL clang/Driver/test/compress.c on AIX (PR #87269)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Thanks for the information. `CHECK-OPT_GZ` is shared by two RUN lines. The failure is due to `-x assembler`.
```
clang '-###' -c --target=powerpc64-ibm-aix -fintegrated-as -gz -x assembler compress.c  # warning: argument unused during compilation: '-gz' [-Wunused-command-line-argument]
```

Since this is unsupported, `UNSUPPORTED:` is more suitable than `XFAIL: system-aix`.

https://github.com/llvm/llvm-project/pull/87269