build: disable cgo on Windows/amd64

Apparently it is broken.  Disable so that dashboard
will let us see other breakages on Windows.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5477081

src/Make.inc

@@ -142,6 +142,9 @@ endif
 ifeq ($(GOOS),netbsd)  # ... and not on NetBSD
 CGO_ENABLED:=0
 endif
+ifeq ($(GOOS)$(GOARCH),windowsamd64)  # ... and not on Windows/amd64
+CGO_ENABLED:=0
+endif
 endif
 
 # Make environment more standard.