internal/bytealg: mask high bit for riscv64 regabi

internal/bytealg: mask high bit for riscv64 regabi

This CL masks byte params which high bits(~0xff) is unused for riscv64
regabi.
Currently the compiler only guarantees the low bits contains value.

Change-Id: I6dd6c867e60d2143fefde92c866f78c4b007a2f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/402894
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: mzh <mzh@golangcn.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>