go/build: fix boolean negation

!(linux&cgo | darwin&cgo)
= (!linux|!cgo) & (!darwin|!cgo)
= (!linux&!darwin) | !cgo  // new comment
≠ !linux | !darwin | !cgo  // old comment

Fixes #3726.

R=golang-dev, bsiegert
CC=golang-dev
https://golang.org/cl/6306076

src/pkg/go/build/doc.go

@@ -100,7 +100,7 @@
 // default functionality for other systems, which in this case would
 // carry the constraint:
 //
-//	// +build !linux !darwin !cgo
+//	// +build !linux,!darwin !cgo
 //
 // Naming a file dns_windows.go will cause it to be included only when
 // building the package for Windows; similarly, math_386.s will be included