cmd/cgo: add support for GOARCH=riscv64

Even though GOARCH=riscv64 is not supported by gc yet, it is easy
to make cmd/cgo already support it.

Together with the changes in debug/elf in CL 107339 this e.g. allows
to generate Go type definitions for linux/riscv64 in the
golang.org/x/sys/unix package without using gccgo.

Change-Id: I6b849df2ddac56c8c483eb03d56009669ca36973
Reviewed-on: https://go-review.googlesource.com/110066


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>