llvm: make sure to use ELFv2 ABI on powerpc64

Currently LLVM is more or less set up to use ELFv2, but it still defaults to
ELFv1 in some places. This causes lld to generate broken binaries when used
with LTO.

PR: 269455
Approved by: dim
MFC after: 3 days

(cherry picked from commit a1ffc2fe9ce54a498c410dcab86495569dbaa7cc)