This is needed in addition to CL 110066 in order to be able to generate
Go type definitions for linux/riscv64 in the golang.org/x/sys/unix
package.

Change-Id: I4a27e6424aaea63283b55bd4f73b958b41f29d72
Reviewed-on: https://go-review.googlesource.com/118618
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>