RFR: 8319316: Clarify text around which layouts a linker supports

[Jorn Vernee]

- Add linker note about packed structs.
- Relax language a bit to avoid implying that only listed layouts are supported.

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Commit messages:
 - simplify language
 - - Add linker note about packed structs.

Changes: https://git.openjdk.org/jdk/pull/16485/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16485&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319316
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16485.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16485/head:pull/16485

PR: https://git.openjdk.org/jdk/pull/16485