cmd/cgo: add support for GOARCH=sparc64

cmd/cgo: add support for GOARCH=sparc64

Even though GOARCH=sparc64 is not supported by gc (yet), it is easy to
make cgo already support it.

This e.g. allows to generate Go type definitions for linux/sparc64 in
the golang.org/x/sys/unix package without using gccgo.

Change-Id: I8886c81e7c895a0d93e350d81ed653fb59d95dd8
Reviewed-on: https://go-review.googlesource.com/102555
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>