[release-branch.go1.4] make.bash: set CGO_ENABLED=0

cgo is not supported with 1.4.

In addition to (perhaps instead of) documenting it,
just enforce it in make.*.

Updates Homebrew/homebrew-core#18465

Change-Id: Iefb1860890d9ec525597b6d730e2189110818598
Reviewed-on: https://go-review.googlesource.com/124076


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>