powerpc64*: fix for broken binaries generated by llvm12

[alfredo]

Summary

Selectively reverts LLVM commit 2518433f861fcb877d0a7bdd9aec1aec1f77505a that was pointed as the source of regression on LLVM12.
This affects powerpc64*, making binaries crash with segmentation fault due to bad code generation around "__stack_chk_guard"

Root cause and/or proper fix is under investigation: https://bugs.llvm.org/show_bug.cgi?id=51590

MFC after: 2 days

[dim]

I wouldn't say "reverts LLVM commit 2518433f861fcb877d0a7bdd9aec1aec1f77505a", but something like "amends LLVM commit 2518433f861fcb877d0a7bdd9aec1aec1f77505a", as we're adding code here. But otherwise, LGTM!

We need to make sure this lands upstream though, and if it's quick enough it might make it into 13.0.0...

[fbsd-phab_maskray.me]

This is not suitable for the upstream. The root cause behind dso_local needs to be found and fixed.

[alfredo]

In D31698#715220, @fbsd-phab_maskray.me wrote:

This is not suitable for the upstream. The root cause behind dso_local needs to be found and fixed.

@fbsd-phab_maskray.me, agreed. Thanks for looking at it. We can continue discussion on upstream https://bugs.llvm.org/show_bug.cgi?id=51590 and https://reviews.llvm.org/D109090.