[llvm] [PowerPC] Fix mask for __st[d/w/h/b]cx builtins (PR #104453)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: c50b22cef7f7ab7b67e884cda42c74eb19de44d3

https://github.com/llvm/llvm-project/actions/runs/10561846199

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/104453