XCVbitmanip: merge custom and standard builtins

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