Shenandoah: Use shorter instruction sequence for checking if marking in progress

In ShenandoahBarrierSetAssembler::generate_c1_pre_barrier_runtime_stub(), we can use shorter instruction sequence for checking if marking in progress:

    // Is marking still active?
    Address gc_state(thread, in_bytes(ShenandoahThreadLocalData::gc_state_offset()));
    __ ldrb(tmp, gc_state);
- __ mov(rscratch2, ShenandoahHeap::MARKING);
- __ tst(tmp, rscratch2);
- __ br(Assembler::EQ, done);
+ __ tbz(tmp, ShenandoahHeap::MARKING_BITPOS, done);
  
    // Can we store original value in the thread's buffer?
    __ ldr(tmp, queue_index);
    __ cbz(tmp, runtime);