internal/bytealg: fix riscv64 offset names
    
    Vet caught that these were incorrect.
    
    Updates #37022
    
    Change-Id: I7b5cd8032ea95eb8e0729f6a4f386aec613c71d8
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217777
    Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
    Reviewed-by: Cherry Zhang <cherryyz@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>