[llvm] [RISCV] Assert extensions are sorted at compile time. NFCI (PR #77442)

[Luke Lau via llvm-commits]

lukel97 wrote:

> I kind of think we should move the extension lists to tblgen so no one has to sort this table manually. We can probably optimize the string lengths better that way by storing them in a big array with sizes like we do in the asm matcher mnemonic table.

That makes sense, I would be more in favour of that approach. 

https://github.com/llvm/llvm-project/pull/77442