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

[Folkert de Vries via llvm-commits]

folkertdev wrote:

>From the issue:

> the .weak assembler directive is currently not supported by LLVM (although it is documented https://www.ibm.com/docs/en/ssw_aix_71/assembler/assembler_pdf.pdf).
> 
> In contrast, the .weak_definition directive is accepted by the assembly parser, but crashes LLVM later on.
> 
> Therefore it is currently impossible to define weak symbols using inline assembly.

So that might mean that actually `.weak_definition` should be rejected by the parser, and instead `.weak` should create weak externals?

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