RFR: 8273122: ZGC: Load forwarding entries without acquire semantics

[Hao Tang]

JDK-8272138 introduced an unbound membar release() between the object copy and the forwarding table insertion.

Thread A (Relocation):
copy();
release();
cas_forwarding_table();
cas_self_heal();

The membar guarantees that the contents of the forwarded object are ready after a forwarding entry is loaded. Since load_object_content(ref) depends on the result of load_forwarding_table(), load_acquire can be safely changed to a simple load.

Thread B (Remapping/Relocation):
ref = load_forwarding_table(); // acquire (current version) -> relaxed (our proposal)
load_object_content(ref);

Our experiment on heapothesys demonstrates >5% time reduction spent on concurrent mark/relocation on AArch64.

---------
### Progress
- [x] Change must not contain extraneous whitespace
- [x] Commit message must refer to an issue
- [ ] Change must be properly reviewed





### Reviewing
<details><summary>Using <code>git</code></summary>

Checkout this PR locally: \
`$ git fetch https://git.openjdk.java.net/jdk pull/5298/head:pull/5298` \
`$ git checkout pull/5298`

Update a local copy of the PR: \
`$ git checkout pull/5298` \
`$ git pull https://git.openjdk.java.net/jdk pull/5298/head`

</details>
<details><summary>Using Skara CLI tools</summary>

Checkout this PR locally: \
`$ git pr checkout 5298`

View PR using the GUI difftool: \
`$ git pr show -t 5298`

</details>
<details><summary>Using diff file</summary>

Download this PR as a diff file: \
<a href="https://git.openjdk.java.net/jdk/pull/5298.diff">https://git.openjdk.java.net/jdk/pull/5298.diff</a>

</details>

-------------

Commit messages:
 - ZGC: Load forwardee without acquire semantics

Changes: https://git.openjdk.java.net/jdk/pull/5298/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5298&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273122
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5298.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5298/head:pull/5298

PR: https://git.openjdk.java.net/jdk/pull/5298