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

contrib/llvm-project/llvm/lib/CodeGen/TargetLoweringBase.cpp

@@ -1961,7 +1961,8 @@ void TargetLoweringBase::insertSSPDeclarations(Module &M) const {
                                   GlobalVariable::ExternalLinkage, nullptr,
                                   "__stack_chk_guard");
     if (TM.getRelocationModel() == Reloc::Static &&
-        !TM.getTargetTriple().isWindowsGNUEnvironment())
+        !TM.getTargetTriple().isWindowsGNUEnvironment() &&
+	!(TM.getTargetTriple().isPPC64() && TM.getTargetTriple().isOSFreeBSD()))
       GV->setDSOLocal(true);
   }
 }