Consider fuzzing `ppc::DoubleDouble` as well, not just `ieee` formats.

[eddyb]

The obvious benefit of testing/fuzzing is almost entirely limited to IEEE and IEEE-style formats (even odd ones can still exercise code paths common to all IEEE floats - that's how llvm/llvm-project#63895 was found).

But because we're not fuzzing PPC's weird "double double" (i.e. (f64, f64)) format, and the upstream tests are pretty lacking, we're still liable to have random bugs laying around, e.g. rust-lang/rust#83725.