compileopts: fix reversed append in the target file

With this fix, `cflags` in the target JSON files is correctly ordered.
Previously, the cflags of a parent JSON file would come after the ones
in the child JSON file, which makes it hard to override properties in
the child JSON file.

Specifically, this fixes the case where targets/riscv32.json sets
`-march=rv32imac` and targets/esp32c3.json wants to override this using
`-march=rv32imc` but can't do this because its `-march` comes before the
riscv32.json one.

compileopts/target.go

@@ -94,7 +94,7 @@ func (spec *TargetSpec) overrideProperties(child *TargetSpec) {
 					dst.Set(src)
 				case "append", "":
 					// or append the field of child to spec
-					dst.Set(reflect.AppendSlice(src, dst))
+					dst.Set(reflect.AppendSlice(dst, src))
 				default:
 					panic("override mode must be 'copy' or 'append' (default). I don't know how to '" + tag + "'.")
 				}

compileopts/target_test.go

@@ -27,7 +27,7 @@ func TestOverrideProperties(t *testing.T) {
 	base := &TargetSpec{
 		GOOS:             "baseGoos",
 		CPU:              "baseCpu",
-		Features:         []string{"bf1", "bf2"},
+		CFlags:           []string{"-base-foo", "-base-bar"},
 		BuildTags:        []string{"bt1", "bt2"},
 		Emulator:         []string{"be1", "be2"},
 		DefaultStackSize: 42,
@@ -37,7 +37,7 @@ func TestOverrideProperties(t *testing.T) {
 	child := &TargetSpec{
 		GOOS:             "",
 		CPU:              "chlidCpu",
-		Features:         []string{"cf1", "cf2"},
+		CFlags:           []string{"-child-foo", "-child-bar"},
 		Emulator:         []string{"ce1", "ce2"},
 		AutoStackSize:    &childAutoStackSize,
 		DefaultStackSize: 64,
@@ -51,8 +51,8 @@ func TestOverrideProperties(t *testing.T) {
 	if base.CPU != "chlidCpu" {
 		t.Errorf("Overriding failed : got %v", base.CPU)
 	}
-	if !reflect.DeepEqual(base.Features, []string{"cf1", "cf2", "bf1", "bf2"}) {
-		t.Errorf("Overriding failed : got %v", base.Features)
+	if !reflect.DeepEqual(base.CFlags, []string{"-base-foo", "-base-bar", "-child-foo", "-child-bar"}) {
+		t.Errorf("Overriding failed : got %v", base.CFlags)
 	}
 	if !reflect.DeepEqual(base.BuildTags, []string{"bt1", "bt2"}) {
 		t.Errorf("Overriding failed : got %v", base.BuildTags)