While the linux/riscv64 port is not yet upstream, most of the syscalls package
already has linux/riscv64 support and this will make testing and development
easier.
Updates golang/go#27532
Change-Id: I6e505be55fdbb2942fb6e4f34c040e7b82809630
Reviewed-on: https://go-review.googlesource.com/c/sys/+/177799
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
