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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Being incompatible with the platform compiler is the wrong default; we should be compatible by default.  And the platform compiler is gcc for almost actively developed ppc32 targets, so we should just switch the default.  Having a flag in addition to that seems fine if you think there's a chance people will need to control it.

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