cmd/compile: build tags don't respect the go version in go.mod

[OneOfOne]

This mainly applies to dev.typeparams since everything breaks if you use generics and go.mod isn't set to 1.18, however I tested with tip and go1.16 and the same issue happens.

To reproduce:

• go.mod

module x

go 1.15

• main.go

package main

func main() {
        println(Version)
}

• main_go115.go

//go:build !go1.16
// +build !go1.16

package main

const Version = "1.15"

• main_go116.go

//go:build go1.16
// +build go1.16

package main

const Version = "1.16"

[seankhliao]

The build tags are chosen by the version of the toolchain used, not the the go directive (if it was it would be redundant)

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