riscv: memcpy_noalignment: Fold SZREG/BLOCK_SIZE alignment to single andi

riscv: memcpy_noalignment: Fold SZREG/BLOCK_SIZE alignment to single andi

Simplify the alignment steps for SZREG and BLOCK_SIZE multiples. The previous
three-instruction sequences

    addi    a7, a2, -SZREG
    andi    a7, a7, -SZREG
    addi    a7, a7, SZREG

and

    addi    a7, a2, -BLOCK_SIZE
    andi    a7, a7, -BLOCK_SIZE
    addi    a7, a7, BLOCK_SIZE

are equivalent to a single

    andi    a7, a2, -SZREG
    andi    a7, a2, -BLOCK_SIZE

because SZREG and BLOCK_SIZE are powers of two in this context, making the
surrounding addi steps cancel out. Folding to one instruction reduces code
size with identical semantics.

No functional change.

sysdeps/riscv/multiarch/memcpy_noalignment.S: Remove redundant addi around
alignment; keep a single andi for SZREG/BLOCK_SIZE rounding.

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