runtime (gc_boehm.go): fix world already stopped check

This fixes the "gc: world already stopped" check by moving it before gcMarkReachable. Previously the check did not work because gcMarkReachable would hang while waiting for activeTaskLock.

Author: niaow

src/runtime/gc_boehm.go

@@ -48,17 +48,17 @@ func gcInit()
 
 //export tinygo_runtime_bdwgc_callback
 func gcCallback() {
+if hasParallelism && needsResumeWorld {
+// Should never happen, check for it anyway.
+runtimePanic("gc: world already stopped")
+}
+
 // Mark globals and all stacks, and stop the world if we're using threading.
 gcMarkReachable()
 
 // If we use a scheduler with parallelism (the threads scheduler for
 // example), we need to call gcResumeWorld() after scanning has finished.
 if hasParallelism {
-if needsResumeWorld {
-// Should never happen, check for it anyway.
-runtimePanic("gc: world already stopped")
-}
-
 // Note that we need to resume the world after finishing the GC call.
 needsResumeWorld = true
 }