Implements the ratified RISC-V Base Cache Management Operation ISA
Extensions: Zicbom and Zicboz, as described in
Zicbop is implemented in a separate patch due to it requiring a new ASM
operand type to be defined.
As discussed in the relevant issue in the upstream spec
https://github.com/riscv/riscv-CMOs/issues/47, the cbo.* instructions
use the format (rs1) or 0(rs1) for their operand, similar to the AMOs.