riscv: memcpy_noalignment: Reorder to store via a3, then bump a3

riscv: memcpy_noalignment: Reorder to store via a3, then bump a3

Rewrite the copy micro-step from:

    REG_L  a4, 0(a5)
    addi   a3, a3, SZREG
    addi   a5, a5, SZREG
    REG_S  a4, -SZREG(a3)

to:

    REG_L  a4, 0(a5)
    addi   a5, a5, SZREG
    REG_S  a4, 0(a3)
    addi   a3, a3, SZREG

Semantics are unchanged: both read *(a5_old), write *(a3_old), and then
increment a3/a5 by SZREG. memcpy assumes non-overlapping regions, so the
reordering preserves correctness.

No functional change.

Signed-off-by: Yao Zihong <zihong.plct@isrc.iscas.ac.cn>
Reviewed-by: Peter Bergner <bergner@tenstorrent.com>