llvm/powerpc64*: fix broken binaries generated by clang12

Amends 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 by:

https://bugs.llvm.org/show_bug.cgi?id=51590

Reviewed by: dim
MFC after: 2 days
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D31698