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

[Justin Hibbits via cfe-commits]

chmeeedalf wrote:

> ping! @chmeeedalf @nemanjai

I know nothing about the complex ABI, so all I can say is the code looks okay from a structural point, can't say anything to the logic.

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