[clang] [clang] Enable --print-supported-extensions for all targets (PR #66586)

[Balint Cristian via cfe-commits]

cbalint13 wrote:

> Put `-mcpu` aside for now, that's nice, but it's only going to be viable in a world where the backend (tablegen) and frontend (target parsers/ABI info) agree on what is valid and can also be passed to clang.

> So what your patch produces is kinda like `./bin/clang --target <whatever> -mllvm -mattr=+unknown_name` and getting `Don't understand unknown_name, you could have passed . (and we have had customers ask for exactly that sort of option, but there's a reason some can't be passed to clang)
 
> Unfortunately the information of what is valid to be passed to clang is not in the same tablegen you're pulling from. It would be nice, and it's not as far off a goal as it used to be, but we aren't there yet.

@DavidSpickett ,

I see the point with the curated lists vs tablegen backend, thank you much for the explanations.
I was only able to check intel's one (w.r.t to -mcpu), without being aware on aarch64 / riscv world's detail.

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My last (potential) idea that might have a purposeful end out of this effort here:

1. For each curated/legit features printed via ```riscvExtensionsHelp()```, ```AArch64::PrintSupportedExtensions()```,   ```ARM::PrintSupportedExtensions()``` could be there a lookup  within getAllProcessorFeatures() for the human-readable description counterpart ?

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Thanks a lot for your time !







https://github.com/llvm/llvm-project/pull/66586