Based on riscv-go port.

Updates #27532

Change-Id: Ia3aed521d4109e7b73f762c5a3cdacc7cdac430d
Reviewed-on: https://go-review.googlesource.com/c/go/+/204635
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>