unix: provide linux/riscv64 assembly for syscalls
unix: provide linux/riscv64 assembly for syscalls

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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys