printer: constraints formatting (#209)

Uses `buildtag.Format` to format constraints in the assembly and stub file
printers. This will ensure `// + build` and `//go:build` syntax are used
consistent with the current Go version.

Updates #183

printer/stubs_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/mmcloughlin/avo/build"
+	"github.com/mmcloughlin/avo/buildtags"
 	"github.com/mmcloughlin/avo/printer"
 )
 
@@ -26,3 +27,32 @@ func TestStubsPragmas(t *testing.T) {
 		"",
 	})
 }
+
+func TestStubsConstraints(t *testing.T) {
+	ctx := build.NewContext()
+	ctx.ConstraintExpr("linux darwin")
+	ctx.ConstraintExpr("amd64 arm64 mips64x ppc64x")
+
+	expect := []string{
+		"// Code generated by avo. DO NOT EDIT.",
+		"",
+	}
+	if buildtags.GoBuildSyntaxSupported() {
+		expect = append(expect,
+			"//go:build (linux || darwin) && (amd64 || arm64 || mips64x || ppc64x)",
+		)
+	}
+	if buildtags.PlusBuildSyntaxSupported() {
+		expect = append(expect,
+			"// +build linux darwin",
+			"// +build amd64 arm64 mips64x ppc64x",
+		)
+	}
+	expect = append(expect,
+		"",
+		"package printer",
+		"",
+	)
+
+	AssertPrintsLines(t, ctx, printer.NewStubs, expect)
+}