Repository: go
Issue: go/build: BuildTags should reject tags with the same name as a known os or arch
Labels: NeedsInvestigation

<!-- Please answer these questions before submitting your issue. Thanks! -->

### What version of Go are you using (`go version`)?

<pre>
$ go version
go version go1.16.3 linux/amd64
</pre>

### Does this issue reproduce with the latest release?

Yes

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/manlio/.local/bin"
GOCACHE="/home/manlio/.cache/go-build"
GOENV="/home/manlio/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE="*.local"
GOMODCACHE="/home/manlio/.local/lib/go/pkg/mod"
GONOPROXY=""
GONOSUMDB="*.local"
GOOS="linux"
GOPATH="/home/manlio/.local/lib/go:/home/manlio/src/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.3"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2654897084=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.16.3 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.16.3
uname -sr: Linux 5.11.11-arch1-1
/usr/lib/libc.so.6: GNU C Library (GNU libc) release release version 2.33.
gdb --version: GNU gdb (GDB) 10.1
</pre></details>

### What did you do?

`go vet -tags=windows` on a Linux host.

### What did you expect to see?

An error, explaining that `"windows"` should not be used as a build tag.


### What did you see instead?

```
# runtime/internal/sys
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:7:7: GOOS redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:8:14
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:9:7: GoosAix redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:10:17
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:10:7: GoosAndroid redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:11:21
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:11:7: GoosDarwin redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:12:20
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:12:7: GoosDragonfly redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:13:23
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:13:7: GoosFreebsd redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:14:21
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:14:7: GoosHurd redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:15:18
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:15:7: GoosIllumos redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:16:21
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:16:7: GoosIos redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:17:17
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:17:7: GoosJs redeclared in this block
	previous declaration at /usr/lib/go/src/runtime/internal/sys/zgoos_linux.go:18:16
/usr/lib/go/src/runtime/internal/sys/zgoos_windows.go:17:7: too many errors
```

A similar error is reported when a different arch is specified as a build tag.

Is there a reason why a build tag with the same name of an os or arch is not rejected?

Thanks.