RFR: 8338727: RISCV: Avoid synthetic data dependency in nmethod barrier on Ztso

[Robbin Ehn]

Hi please consider,

On TSO we don't need the synthetic data dependency in between the loads.
Also added some comment about this.

Sanity tested

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

Commit messages:
 - 8338727: RISCV: Avoid synthetic data dependency in nmethod barrier on Ztso

Changes: https://git.openjdk.org/jdk/pull/20661/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20661&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338727
  Stats: 12 lines in 1 file changed: 7 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/20661.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20661/head:pull/20661

PR: https://git.openjdk.org/jdk/pull/20661