Only exe buildmode without cgo supported.

Change-Id: Id104a79a99d3285c04db00fd98b8affa94ea3c37
Reviewed-on: https://go-review.googlesource.com/31487
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>