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:
Reviewed by: dim
MFC after: 2 days
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D31698
(cherry picked from commit 9a4d48a645a7a3ebee05fae25afd154a132b638a)