[llvm] [AArch64] Define APAS, BRB and TRCIT as SYS aliases (PR #203563)

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https://github.com/CarolineConcatto commented:

Thank you for splitting the patch. It makes easier to review now.
I left some questions about brb. But I think the patch looks better now.

https://github.com/llvm/llvm-project/pull/203563