// sets the feature build tags for all previous levels as well.
// For example, GOAMD64=v2 sets the amd64.v1 and amd64.v2 feature flags.
// This ensures that code making use of v2 features continues to compile
// when, say, GOAMD64=v4 is introduced.
// Code handling the absence of a particular feature level
// should use a negation:
//
//	//go:build !amd64.v2
//