Shenandoah: Claim verifier thread roots for parallel processing

We see occasional crashes during CI testing:

# Internal Error (/home/jenkins/workspace/nightly/jdk-jdk/src/hotspot/cpu/x86/frame_x86.cpp:684), pid=25982, tid=26009
# assert(_last_Java_pc == NULL) failed: already walkable

Stack: [0x00007f5d0e9e6000,0x00007f5d0eae6000], sp=0x00007f5d0eae1820, free space=1006k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0xa1b474] frame::sender_for_entry_frame(RegisterMap*) const+0x194
V [libjvm.so+0xa1c2f8] frame::sender(RegisterMap*) const+0x228
V [libjvm.so+0x16856ac] JavaThread::oops_do(OopClosure*, CodeBlobClosure*)+0x3ac
V [libjvm.so+0x1690683] Threads::possibly_parallel_oops_do(bool, OopClosure*, CodeBlobClosure*)+0x133
V [libjvm.so+0x1514f6d] ShenandoahRootVerifier::roots_do(OopClosure*)+0x23d
V [libjvm.so+0x1540052] ShenandoahVerifierReachableTask::work(unsigned int)+0x242
V [libjvm.so+0x17b9f74] GangWorker::run_task(WorkData)+0x84
V [libjvm.so+0x17ba0b8] GangWorker::loop()+0x48
V [libjvm.so+0x1691cb6] Thread::call_run()+0xf6
V [libjvm.so+0x12bce56] thread_native_entry(Thread*)+0x116