Go 1.4 should build what it knows how to build.
GOHOSTOS and GOHOSTARCH are for the Go 1.5 build only.

Change-Id: Id0f367f03485100a896e61cfdace4ac44a22e16d
Reviewed-on: https://go-review.googlesource.com/2818Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>