[release-branch.go1.4] go/build: don't enable cgo for any system by default

The 1.4 branch is now used only for building more recent releases.
Enabling cgo is not required for that, and tends to cause problems as
systems and compilers change.

Updates #21054

Change-Id: I498f493e3ac32bcea55725cf5de3b3172f0fd5db
Reviewed-on: https://go-review.googlesource.com/65450


Reviewed-by: Joe Tsai <joetsai@google.com>
Run-TryBot: Joe Tsai <joetsai@google.com>