riscv: Add Zbkb optimized repeat_bytes helper

riscv: Add Zbkb optimized repeat_bytes helper

Introduce a RISC-V specific string-misc.h to provide an optimized
repeat_bytes implementation when the Zbkb extension is available.
The new version uses packh/packw/pack instruction count and avoiding
high latency instructions. This helper is used by several mem and string
functions, and falls back to the generic implementation when Zbkb is not
present.

Signed-off-by: Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>
Reviewed-by: Peter Bergner <bergner@tenstorrent.com>