[clang] [clang][PowerPC] Add flag to enable compatibility with GNU for complex arguments (PR #77732)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

`-std=gnu99` etc. sets LangOpts.GNUMode... but we don't want to base the ABI on that; it's not an ABI flag.  The ABI should only be based on the triple, and flags specifically intended to modify the ABI.

I think we should just use the GNU-compatible ABI by default.  Unless there's some existing OS where clang is the platform compiler that wants to preserve the old ABI, but even if such a platform exists, I doubt there's anyone we can ask.

https://github.com/llvm/llvm-project/pull/77732