cmd/dist: fix default GOOS/GOARCH for cross-compile
Before this CL, if you are on a darwin/amd64 machine and
cross-compile 9g for a linux/ppc64 machine, when you copy
9g over to that kind of machine and run it, you'll find it thinks
the default object target is darwin/amd64. Not useful.
Make the default target linux/ppc64 in this case. More useful.
Change-Id: I62f2e9cb5f60b3077a922b31cd023a9cb7a6cfda
Reviewed-on: https://go-review.googlesource.com/6407 Reviewed-by: Rob Pike <r@golang.org>