cmd/dist: implement //go:build parsing

The bootstrap directories are built with the Go 1.4 go command,
and they will retain the // +build lines until we bump the bootstrap
toolchain to Go 1.17 or later.

cmd/dist builds cmd/go and all its dependencies, using the
assembler, compiler, and linker that were built using Go 1.4.
We don't want to have to keep // +build lines in cmd/go and
all its dependencies, so this CL changes cmd/dist to understand
the //go:build lines.

cmd/dist is a standalone Go program that must itself build with
very old Go releases, so we cannot assume go/build/constraint
is available. Instead, implement a trivial parser/evaluator.

For #41184.

Change-Id: I84e259dec3bd3daec3f82024eb3500120f53096d
Reviewed-on: https://go-review.googlesource.com/c/go/+/359314


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>