[riscv] Speed up memmove() when copying in forwards direction

[riscv] Speed up memmove() when copying in forwards direction

Use the word-at-a-time variable-length memcpy() implementation when
performing an overlapping copy in the forwards direction, since this
is guaranteed to be safe and likely to be substantially faster than
the existing bytewise copy.

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