Shenandoah: relax update references memory ordering even more

Shenandoah evacuates object and then updates the reference in load barriers. Currently, memory order release is adopted in atomic_update_oop() (done in JDK-8261495), and we propose to use relaxed instead. Since memory order conservative is adopted in updating forwardee, this membar ensures that object copy is finished before updating the reference. In the scenario when a thread reads self-healed address from forwardee, relaxed is also fine due to the data dependency of the self-healed address.
This relaxation of memory order brings us some performance improvement. We have run specjbb2015 on AArch64. Critical-JOPS increases by 3% while max-JOPS maintains almost the same.
```
baseline_1:RUN RESULT: hbIR (max attempted) = 122581, hbIR (settled) = 107513, max-jOPS = 102968, critical-jOPS = 37913
baseline_2:RUN RESULT: hbIR (max attempted) = 122581, hbIR (settled) = 106769, max-jOPS = 101742, critical-jOPS = 37151
baseline_3:RUN RESULT: hbIR (max attempted) = 122581, hbIR (settled) = 110118, max-jOPS = 101742, critical-jOPS = 37041

optimized_1:RUN RESULT: hbIR (max attempted) = 122581, hbIR (settled) = 111708, max-jOPS = 101742, critical-jOPS = 37687
optimized_2:RUN RESULT: hbIR (max attempted) = 147077, hbIR (settled) = 122581, max-jOPS = 104425, critical-jOPS = 39663
optimized_3:RUN RESULT: hbIR (max attempted) = 122581, hbIR (settled) = 107286, max-jOPS = 102968, critical-jOPS = 38579
```