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

[David Spickett via cfe-commits]

DavidSpickett wrote:

> I was only able to check intel's one (w.r.t to -mcpu), without being aware on aarch64 / riscv world's detail.

Yes I wouldn't expect you to right away, that's why we all keep an eye out for each other's changes :) . That's what I meant about starting by writing more comprehensive testing for the existing supported architectures, it would help you fully understand what the existing feature does.

> could be there a lookup within getAllProcessorFeatures() for the human-readable description counterpart ?

Without seeing at least a sketch of what that would look like I can't say. Could we for example move some of this "frontend" information back into the backends, yes. That's not the unknown there, it's the how. So it's best just to try it yourself and maybe put up a draft PR if you think it can be done.

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