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

[Fangrui Song via llvm-commits]

https://github.com/MaskRay commented:

Do you need .weak_definition? ELF doesn't support .weak_definition and we  don't use MCSA_WeakDefinition for ELF. You can use MCSA_Weak for XCOFF

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