[llvm] Store sysreg names in-line with their descriptors. (PR #119157)

[Sam Elliott via llvm-commits]

lenary wrote:

How good is the error checking, if a future RISC-V extension adds a sysreg with a longer name? Do we need to find a place for a build-time error with a nice message?

https://github.com/llvm/llvm-project/pull/119157