go/build: fix boolean negation
go/build: fix boolean negation

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

Fixes issue 3726.

R=golang-dev, bsiegert
CC=golang-dev
http://codereview.appspot.com/6306076

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