cmd/cgo, cmd/compile, cmd/link: remove old style build tags

cmd/cgo, cmd/compile, cmd/link: remove old style build tags

[Roll-forward of CL 436915 by Tobias Klauser, with builtin and gen
directories dropped now that they've been handled separately.]

The minimum bootstrap version for Go ≥ 1.20 is Go 1.17. That version
supports the new style //go:build lines. Thus the old style //+build
lines can be dropped in this part of the tree as well. Leave the
//+build lines in cmd/dist which will ensure the minimum Go version
during bootstrap.

As suggested by Cherry during review of CL 430496

For #44505

Change-Id: Ifa686656c3e50bf7f92f70747b44d74a7d51bad8
Reviewed-on: https://go-review.googlesource.com/c/go/+/435473
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>