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

[Alex Bradbury via llvm-commits]

asb wrote:

I have a slight concern that one of the compilers that LLVM is meant to be buildable by is unhappy with the use of `constexpr`. Sadly MSVC 16.7 (apparently the [oldest supported version](https://llvm.org/docs/GettingStarted.html#check-here)) doesn't seem to be available on godbolt though :/

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