ast,build: add build constraints to File

Updates #3

printer/goasm_test.go

@@ -67,3 +67,17 @@ func TestTextDecl(t *testing.T) {
 		"",
 	})
 }
+
+func TestConstraints(t *testing.T) {
+	ctx := build.NewContext()
+	ctx.BuildConstraintExpr("linux,386 darwin,!cgo")
+	ctx.BuildConstraintExpr("!noasm")
+
+	AssertPrintsLines(t, ctx, printer.NewGoAsm, []string{
+		"// Code generated by avo. DO NOT EDIT.",
+		"",
+		"// +build linux,386 darwin,!cgo",
+		"// +build !noasm",
+		"",
+	})
+}