arch/riscv/lib: update memmove and memcpy for big-endian

arch/riscv/lib: update memmove and memcpy for big-endian

Change the shift patterns for the unaligned memory move and copy code
to deal with big-endian by definign macros to change the shfit left and
right to go the opposite way.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>