Go1.4 does not recognize arm64 and ppc64le as valid GOARCH values, so
we need explicit build tags.

Change-Id: I1a886c132ae398d9f92b2c2b33d2a4827bbdb9d4
Reviewed-on: https://go-review.googlesource.com/10083Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>