go.mod: update go.mod for riscv64

Fixes golang/go#48355

Change-Id: I04d746dfe00bc4ef3e4281aca35441250dbf36c8
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/349589
Trust: Meng Zhuo <mzh@golangcn.org>
Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
2 files changed