[riscv] Add optimised TCP/IP checksumming

[riscv] Add optimised TCP/IP checksumming

Add a RISC-V assembly language implementation of TCP/IP checksumming,
which is around 50x faster than the generic algorithm.  The main loop
checksums aligned xlen-bit words, using almost entirely compressible
instructions and accumulating carries in a separate register to allow
folding to be deferred until after all loops have completed.

Experimentation on a C910 CPU suggests that this achieves around four
bytes per clock cycle, which is comparable to the x86 implementation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>