ShenandoahSafepoint::is_at_shenandoah_safepoint should not access VMThread state from other threads

The following tests fail with debug VMs:

compiler/c1/TestLinearScanOrderMain.java
gc/CriticalNativeArgs.java#id1
gc/TestHumongousReferenceObject.java#id2
gc/TestSystemGC.java#id3
gc/class_unloading/TestClassUnloadingDisabled.java#id3
gc/ergonomics/TestDynamicNumberOfGCThreads.java
gc/logging/TestGCId.java
gc/metaspace/TestMetaspacePerfCounters.java#id3
gc/shenandoah/TestSmallHeap.java
gc/shenandoah/compiler/TestC1ArrayCopyNPE.java
gc/shenandoah/compiler/TestC1VectorizedMismatch.java
gc/shenandoah/compiler/TestClone.java#id0
gc/shenandoah/compiler/TestClone.java#id1
gc/shenandoah/compiler/TestClone.java#id2
gc/shenandoah/compiler/TestClone.java#id3
gc/shenandoah/compiler/TestClone.java#id4
gc/shenandoah/compiler/TestClone.java#id5
gc/shenandoah/compiler/TestReferenceCAS.java#id0
gc/shenandoah/compiler/TestReferenceCAS.java#id1
gc/shenandoah/mxbeans/TestChurnNotifications.java#id0
gc/shenandoah/mxbeans/TestChurnNotifications.java#id1
gc/shenandoah/mxbeans/TestChurnNotifications.java#id2
gc/shenandoah/mxbeans/TestChurnNotifications.java#id3
gc/shenandoah/mxbeans/TestChurnNotifications.java#id4
gc/shenandoah/mxbeans/TestChurnNotifications.java#id5
gc/shenandoah/mxbeans/TestPauseNotifications.java#id0
gc/shenandoah/mxbeans/TestPauseNotifications.java#id1
gc/shenandoah/mxbeans/TestPauseNotifications.java#id2
gc/shenandoah/mxbeans/TestPauseNotifications.java#id3
gc/shenandoah/mxbeans/TestPauseNotifications.java#id4
gc/shenandoah/mxbeans/TestPauseNotifications.java#id5
gc/shenandoah/oom/TestAllocLargeObj.java
gc/shenandoah/oom/TestAllocLargerThanHeap.java
gc/shenandoah/oom/TestAllocSmallObj.java
gc/shenandoah/oom/TestClassLoaderLeak.java
gc/shenandoah/oom/TestThreadFailure.java
gc/shenandoah/options/TestArgumentRanges.java
gc/shenandoah/options/TestExplicitGC.java
gc/shenandoah/options/TestHeuristicsUnlock.java
gc/shenandoah/options/TestHumongousMoves.java
gc/shenandoah/options/TestPacing.java
gc/shenandoah/options/TestParallelRegionStride.java
gc/shenandoah/options/TestSingleThreaded.java
gc/shenandoah/options/TestSoftMaxHeapSize.java
gc/stress/CriticalNativeStress.java#id1
java/lang/invoke/RicochetTest.java


# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/jdk/src/hotspot/share/runtime/vmThread.hpp:103), pid=2517, tid=2564
# assert(Thread::current()->is_VM_thread()) failed: Must be
#
# JRE version: OpenJDK Runtime Environment (16.0) (fastdebug build 16-internal+0-adhoc..jdk)