Fix detach_all_threads_and_tear_down_isolate

Ensure `DetachAllExternallyStartedThreadsExceptCurrentOperation` doesn't
detach the thread starting the operation as that may leave it in an
inconsistent state. `leaveTearDownIsolate` will properly teardown/detach
both the operation thread (if there is one) and the current thread.

Author: gilles-duboscq

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/thread/VMThreads.java

@@ -688,11 +688,11 @@ private static class DetachAllExternallyStartedThreadsExceptCurrentOperation ext
 
         @Override
         protected void operate() {
-            IsolateThread currentThread = CurrentIsolate.getCurrentThread();
+            IsolateThread operationThread = CurrentIsolate.getCurrentThread();
             IsolateThread thread = firstThread();
             while (thread.isNonNull()) {
                 IsolateThread next = nextThread(thread);
-                if (thread.notEqual(currentThread) && !wasStartedByCurrentIsolate(thread)) {
+                if (thread.notEqual(queuingThread) && thread.notEqual(operationThread) && !wasStartedByCurrentIsolate(thread)) {
                     /*
                      * The code below is similar to VMThreads.detachCurrentThread() except that it
                      * doesn't call VMThreads.threadExit(). We assume that this is tolerable