internal/cpu: fix wrong cache line size of riscv64

internal/cpu: fix wrong cache line size of riscv64

All of riscv CPU using 64B for cache-line size.
i.e. U540 of Hifive Unleashed (https://www.sifive.com/boards/hifive-unleashed)

Change-Id: I0d72d88ac026f45383c3b3eb3a77233d3c2e4004
Reviewed-on: https://go-review.googlesource.com/c/go/+/526659
Run-TryBot: M Zhuo <mzh@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>