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/goasm_test.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/mmcloughlin/avo/attr"
 	"github.com/mmcloughlin/avo/build"
+	"github.com/mmcloughlin/avo/buildtags"
 	"github.com/mmcloughlin/avo/printer"
 	"github.com/mmcloughlin/avo/reg"
 )
@@ -73,13 +74,24 @@ func TestConstraints(t *testing.T) {
 	ctx.ConstraintExpr("linux,386 darwin,!cgo")
 	ctx.ConstraintExpr("!noasm")
 
-	AssertPrintsLines(t, ctx, printer.NewGoAsm, []string{
+	expect := []string{
 		"// Code generated by avo. DO NOT EDIT.",
 		"",
-		"// +build linux,386 darwin,!cgo",
-		"// +build !noasm",
-		"",
-	})
+	}
+	if buildtags.GoBuildSyntaxSupported() {
+		expect = append(expect,
+			"//go:build ((linux && 386) || (darwin && !cgo)) && !noasm",
+		)
+	}
+	if buildtags.PlusBuildSyntaxSupported() {
+		expect = append(expect,
+			"// +build linux,386 darwin,!cgo",
+			"// +build !noasm",
+		)
+	}
+	expect = append(expect, "")
+
+	AssertPrintsLines(t, ctx, printer.NewGoAsm, expect)
 }
 
 func TestAlignmentNoOperands(t *testing.T) {