[clang] [llvm] [RISC-V][RVY] Fix round-trip and order of arch strings with 'y' (PR #227865)

[Alexander Richardson via llvm-commits]

arichardson wrote:

@jrtc27 thanks for pointing out the `.attribute arch` problem. I've updated it to fix round-tripping. We now always emit `rv64i2p1y0p910` which will allow CHERIoT to emit `rv32e2p1y0p910`. 

This aligns with the long base name proposal and the ARC response to our query: for RVY Integer mode we should just use rv64i_zyhybrid and omit the y base since it behaves like RVI.

I believe your changes always omit i from the arch string but still has it implied. I think this current approach is slightly more consistent since the normalized arch string now  includes all implied extensions again.

https://github.com/llvm/llvm-project/pull/227865