[lldb][AIX] Some base #if _AIX changes of a minimal lldb build (#120979)

This PR is in reference to porting LLDB on AIX.

Link to discussions on llvm discourse and github:

1. https://discourse.llvm.org/t/port-lldb-to-ibm-aix/80640
2. https://github.com/llvm/llvm-project/issues/101657
The complete changes for porting are present in this draft PR:
https://github.com/llvm/llvm-project/pull/102601

Added some base #if _AIX changes for minimal lldb build.
Added a PR for clang-format changes separately, to rebase this on later:

- https://github.com/llvm/llvm-project/pull/120978

Review Request: @labath @DavidSpickett

GitOrigin-RevId: 18de1db0cfbfbbf12d16338923b43077a87dce18
3 files changed