Shenandoah GC specific safepoints are scheduled by control thread, therefore, query from control thread should return false.

I saw an assertion failure on x86_32, due to the wrong answer.

# Internal Error (/home/zgu/ws/jdk/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp:2036), pid=1410312, tid=1410328
# assert(nworkers == ParallelGCThreads) failed: Use ParallelGCThreads within safepoints

Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x14c3977] ShenandoahHeap::assert_gc_workers(unsigned int)+0x157
V [libjvm.so+0x1497a9e] ShenandoahConcurrentGC::entry_evacuate()+0x8e
V [libjvm.so+0x149a4de] ShenandoahConcurrentGC::collect(GCCause::Cause)+0x28e
V [libjvm.so+0x14ac296] ShenandoahControlThread::service_concurrent_normal_cycle(GCCause::Cause)+0x96
V [libjvm.so+0x14acddd] ShenandoahControlThread::run_service()+0x74d
V [libjvm.so+0x9455c6] ConcurrentGCThread::run()+0x26
V [libjvm.so+0x16c602a] Thread::call_run()+0xfa
V [libjvm.so+0x12cd3d7] thread_native_entry(Thread*)+0x137
C [libpthread.so.0+0x867e] start_thread+0xee