[llvm] [PowerPC][AIX] Emit weak_definition symbols as weak externals (PR #156072)

[Hubert Tong via llvm-commits]

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hubert-reinterpretcast wrote:

I don't see why we wouldn't just do what ELF does, which is to return `false` from `emitSymbolAttribute`.

https://github.com/llvm/llvm-project/pull/156072