[vm, gc] Fix race between BlockForSafepoint and TryStealActiveMutator.

This race can result in the stolen bit becoming cleared and the same thread stolen from multiple tests.

TEST=vm/dart/isolates/many_isolates_blocked_at_process_run_sync_test
Change-Id: I737458ff48e9d01ddfe4e5a016ef57e4cbdbf292
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425740
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/thread.h

@@ -1021,8 +1021,12 @@ class Thread : public ThreadState {
   }
   void SetBlockedForSafepoint(bool value) {
     ASSERT(thread_lock()->IsOwnedByCurrentThread());
-    safepoint_state_ =
-        BlockedForSafepointField::update(value, safepoint_state_);
+    const uword mask = BlockedForSafepointField::mask_in_place();
+    if (value) {
+      safepoint_state_.fetch_or(mask);
+    } else {
+      safepoint_state_.fetch_and(~mask);
+    }
   }
   bool BypassSafepoints() const {
     return BypassSafepointsField::decode(safepoint_state_);