[clang] [PowerPC] make `_Complex` ABI GCC-compatible (PR #208917)

[Folkert de Vries via cfe-commits]

folkertdev wrote:

Updated this again, and copied over a bunch of tests that were added for sparc to lock in the generated IR for all types.

Periodic ping @amy-kwan @daltenty @lei137 I'll re-iterate that for rust we would really like to see clang doing the right thing here, i.e. not miscompile even basic usage of the div/mul libcalls.

https://github.com/llvm/llvm-project/pull/208917