[clang] [llvm] [RISCV] Add generic CPUs for profiles (PR #84877)

[Alex Bradbury via cfe-commits]

asb wrote:

We discussed again in the sync-up call to try to move this forwards and per a discussion with @topperc and @preames were wondering if relying on -mattr and a corresponding feature for the profile might be the better approach after all.
* There's not a lot of instances of this, but I do see at least some ARM tests using -mattr=+armv8.2-a or +armv8.3-a. So there's some precedent there.
  * Looking again, x86 does indeed use -mcpu in llc for x86_64-v3 etc. So that could be an argument for following suit, but for some reason it still feels like defining new generic CPUs isn't very consistent with how we normally specific extensions in RISC-V. So -march for clang and -mattr for llc feels consistent to me.
* Philip raised the point that if we're making this change primarily to ease usage of llc, it's unfortunate if we're adding something that is exposed to clang that we wouldn't necessarily want otherwise (in this case, the generic cpu for the profile).

That said, it was a brief discussion and we may have missed something or Philip or Craig may have more thoughts afterwards.

Do you see any disadvantage to going back to adding a feature for the profiles?

https://github.com/llvm/llvm-project/pull/84877