riscv: Add checksum header

Provide checksum algorithms that have been designed to leverage riscv
instructions such as rotate. In 64-bit, can take advantage of the larger
register to avoid some overflow checking.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Xiao Wang <xiao.w.wang@intel.com>
Link: https://lore.kernel.org/r/20240108-optimize_checksum-v15-3-1c50de5f2167@rivosinc.com


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