[FMV] Document feature dependencies and detect at selection.

[labrinea]

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[labrinea]

@jroelofs @andrewcarlotti @Wilco1

[andrewcarlotti]

Last time I checked, I don't think any part of the toolchains has an fp16fml->simd dependency for command line arguments, so this seems inconsistent.

[andrewcarlotti]

In GCC+Bintuils we only have bf16 depending on fp, and in LLVM it depends on nothing at all. I think fp is more appropriate than simd.

[andrewcarlotti]

This should probably have a simd dependency, and perhaps also an fcma dependency?

[andrewcarlotti]

This doesn't currently match existing command line options, but those should probably be adjusted. Should this also have a simd and perhaps fcma dependency?

[andrewcarlotti]

I don't agree with this - while it's unlikely to come up in practice, I think the selection algorithm would be simpler to understand if it were based solely upon the attribute strings and the feature priorities, without trying to detect and ignore any redundant dependencies.