RISC-V: replace cbom instructions with an insn-def

Using the cbom instructions directly in ALT_CMO_OP, requires toolchain
support for the instructions. Using an insn-def will allow for removal
of toolchain version checks in the build system & simplification of the
proposed [1] function-based CMO scheme.

Link: https://lore.kernel.org/linux-riscv/fb3b34ae-e35e-4dc2-a8f4-19984a2f58a8@app.fastmail.com/

 [1]
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230108163356.3063839-3-conor@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>