8237080: fatal error: VM thread could block on lock that may be held …

…by a JavaThread during safepoint: SharedDecoder_lock

Set allow_vm_block to true for this lock.  It's _safepoint_check_never so it's sort of implied (you can't safepoint holding the lock and block out the vm thread).

Reviewed-by: rehn, zgu, pchilanomate, dholmes

src/hotspot/share/runtime/mutexLocker.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -329,7 +329,7 @@ void mutex_init() {
   def(NMethodSweeperStats_lock     , PaddedMutex  , special,     true,  _safepoint_check_never);
   def(ThreadsSMRDelete_lock        , PaddedMonitor, special,     true,  _safepoint_check_never);
   def(ThreadIdTableCreate_lock     , PaddedMutex  , leaf,        false, _safepoint_check_always);
-  def(SharedDecoder_lock           , PaddedMutex  , native,      false, _safepoint_check_never);
+  def(SharedDecoder_lock           , PaddedMutex  , native,      true,  _safepoint_check_never);
   def(DCmdFactory_lock             , PaddedMutex  , leaf,        true,  _safepoint_check_never);
 #if INCLUDE_NMT
   def(NMTQuery_lock                , PaddedMutex  , max_nonleaf, false, _safepoint_check_always);