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

[Owen Anderson via llvm-commits]

resistor 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?

The error checking seems fine to me:

```
/Users/resistor/llvm-project/build-release/lib/Target/RISCV/RISCVGenSearchableTables.inc:34559:5: error: initializer-string for char array is too long, array size is 32 but initializer has size 49 (including the null terminating character)
 34559 |   { "testtesttesttesttesttesttesttesttesttesttesttest", "testtesttesttesttesttesttesttesttesttesttesttest", "", 0x283,  {} , false }, // 64
       |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

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