wasm: backport "garbage collect references to JavaScript values"

See commit:
https://github.com/golang/go/commit/54e6ba6724dfde355070238f9abc16362cac2e3d

Warning: this will drop support for Go 1.13 for WebAssembly targets!
I have modified the integration tests to specifically blacklist Go 1.13
instead of whitelisting any other version, to avoid accidentally not
testing WebAssembly.

main_test.go

@@ -13,12 +13,14 @@ import (
 	"path/filepath"
 	"runtime"
 	"sort"
+	"strings"
 	"sync"
 	"testing"
 	"time"
 
 	"github.com/tinygo-org/tinygo/builder"
 	"github.com/tinygo-org/tinygo/compileopts"
+	"github.com/tinygo-org/tinygo/goenv"
 )
 
 const TESTDATA = "testdata"
@@ -71,9 +73,20 @@ func TestCompiler(t *testing.T) {
 		t.Run("ARM64Linux", func(t *testing.T) {
 			runPlatTests("aarch64--linux-gnu", matches, t)
 		})
-		t.Run("WebAssembly", func(t *testing.T) {
-			runPlatTests("wasm", matches, t)
-		})
+		goVersion, err := builder.GorootVersionString(goenv.Get("GOROOT"))
+		if err != nil {
+			t.Error("could not get Go version:", err)
+			return
+		}
+		minorVersion := strings.Split(goVersion, ".")[1]
+		if minorVersion != "13" {
+			// WebAssembly tests fail on Go 1.13, so skip them there. Versions
+			// below that are also not supported but still seem to pass, so
+			// include them in the tests for now.
+			t.Run("WebAssembly", func(t *testing.T) {
+				runPlatTests("wasm", matches, t)
+			})
+		}
 	}
 }