RFR: 8321631: Fix comments in access.hpp

[Guoxiong Li]

Hi all,

This trivial patch fixes the comments about `atomic_xchg` and `atomic_xchg_at` in `access.hpp`.
And it removes the unnecessary content about `INSTANTIATE_HPP_ACCESS` 
which has been aleready removed in [JDK-8230808](https://bugs.openjdk.org/browse/JDK-8230808).

Thanks for the review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8321631

Changes: https://git.openjdk.org/jdk/pull/17042/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17042&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321631
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17042.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17042/head:pull/17042

PR: https://git.openjdk.org/jdk/pull/17042