[llvm-branch-commits] [llvm] [release/20.x] Update release notes for SystemZ changes (PR #140060)

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github-actions[bot] wrote:

@uweigand (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix.  When you are done, please add the release:note label to this PR. 

https://github.com/llvm/llvm-project/pull/140060