[BPF][Test] Disable codegen test on AIX

https://reviews.llvm.org/D101194 changed the default getMultiarchTriple in toolchain.
So -march=bpf on AIX will get triple of bpf-ibm-aix now,
this is unexpected and causing test failures.

BPF on AIX is not supported (yet), disable the codegen test on AIX in lit cfg.

Reviewed By: yonghong-song

Differential Revision: https://reviews.llvm.org/D101866

llvm/test/CodeGen/BPF/lit.local.cfg

@@ -1,2 +1,4 @@
 if not 'BPF' in config.root.targets:
     config.unsupported = True
+if 'system-aix' in config.available_features:
+    config.unsupported = True