Add -gc=custom option (#3302)

* Add -gc=custom option

compileopts/config.go

@@ -105,7 +105,7 @@ func (c *Config) GC() string {
 // that can be traced by the garbage collector.
 func (c *Config) NeedsStackObjects() bool {
 	switch c.GC() {
-	case "conservative", "precise":
+	case "conservative", "custom", "precise":
 		for _, tag := range c.BuildTags() {
 			if tag == "tinygo.wasm" {
 				return true

compileopts/options.go

@@ -8,7 +8,7 @@ import (
 )
 
 var (
-	validGCOptions            = []string{"none", "leaking", "conservative", "precise"}
+	validGCOptions            = []string{"none", "leaking", "conservative", "custom", "precise"}
 	validSchedulerOptions     = []string{"none", "tasks", "asyncify"}
 	validSerialOptions        = []string{"none", "uart", "usb"}
 	validPrintSizeOptions     = []string{"none", "short", "full"}

compileopts/options_test.go

@@ -9,7 +9,7 @@ import (
 
 func TestVerifyOptions(t *testing.T) {
 
-	expectedGCError := errors.New(`invalid gc option 'incorrect': valid values are none, leaking, conservative, precise`)
+	expectedGCError := errors.New(`invalid gc option 'incorrect': valid values are none, leaking, conservative, custom, precise`)
 	expectedSchedulerError := errors.New(`invalid scheduler option 'incorrect': valid values are none, tasks, asyncify`)
 	expectedPrintSizeError := errors.New(`invalid size option 'incorrect': valid values are none, short, full`)
 	expectedPanicStrategyError := errors.New(`invalid panic option 'incorrect': valid values are print, trap`)
@@ -48,6 +48,12 @@ func TestVerifyOptions(t *testing.T) {
 				GC: "conservative",
 			},
 		},
+		{
+			name: "GCOptionCustom",
+			opts: compileopts.Options{
+				GC: "custom",
+			},
+		},
 		{
 			name: "InvalidSchedulerOption",
 			opts: compileopts.Options{