XCVbitmanip: merge custom and standard builtins

Placeholder MaryBennett
2024-09-27T07:59:10Z

As discussed during the review of the XCVbitmanip patch, the custom builtins are very similar to the standard RISC-V bitmanip builtins. The custom builtins should be merged with the standard builtins before upstreaming. This is similar to what was done for XCVbi.

Instructions to be changed:

- [ ] bclr
- [ ] bset
- [ ] ff1
- [ ] fl1
- [ ] clb
- [ ] cnt
- [ ] ror
- [ ] bitrev