Fix the logic for scanning tasks in the Julia GC.

This removes an unnecessary (and wrong) test that intended to avoid
repeated scanning of the main thread stack, but instead skipped
scanning of all root stacks.

src/julia_gc.c

@@ -646,7 +646,7 @@ static void GapTaskScanner(jl_task_t * task, int root_task)
         if (tag->bits.gc & 2)
             rescan = 0;
     }
-    if (stack && tid < 0) {
+    if (stack) {
         if (task->copy_stack) {
             // We know which part of the task stack is actually used,
             // so we shorten the range we have to scan.