[AIX] Add system-aix to lit config file

Summary: This is a complementary patch to D82100 since the aix builbot is still running the unsupported test shtest-format-argv0. Add system-aix to the sub llvm-lit config.

Reviewers: daltenty, hubert.reinterpretcast

Reviewed By: hubert.reinterpretcast

Subscribers: delcypher, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82905