With cgo supporting sparc64 as of CL 102555 and CL 132155, the Docker
based build system can be used to generate file for linux/sparc64 as
well.

Updates golang/go#15282

Change-Id: I109e55f39d3229b24b0029c42074e439c6c54dc8
Reviewed-on: https://go-review.googlesource.com/c/102655
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>