cmd/go: change +build doc references to //go:build
This changes a few references to `+build` into the modern `//go:build`.
It was compiled by editing `cmd/go/internal/list/context.go`, running
`go test cmd/go -v -run=TestDocsUpToDate -fixdocs`, and then editing
list.go and build.go by hand.
Change-Id: I00fec55e098bf5100f5a186dd975a6628e15beb8
GitHub-Last-Rev:
e0eb9be77e9e409479ef48a65f750546d2a9ddcc
GitHub-Pull-Request: golang/go#59245
Reviewed-on: https://go-review.googlesource.com/c/go/+/479417
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>