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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> `.weak_definition` is now rejected at parse time on XCOFF.

@aabhinavg1, please update the PR description to match the content.

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