devel/llvm12: fix for broken binaries generated by llvm12

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

Submitted by: adalava
Differential review (for base): https://reviews.freebsd.org/D31698