RFR: 8340205: Native linker allows MemoryLayout consisting of only PaddingLayout [v4]

[Per Minborg]

> This PR prevents sequence layout with padding to be used with the Linker.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Add to specification and refine detection of PL GLs

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21041/files
  - new: https://git.openjdk.org/jdk/pull/21041/files/78d38efa..bba0b784

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21041&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21041&range=02-03

  Stats: 6 lines in 3 files changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21041.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21041/head:pull/21041

PR: https://git.openjdk.org/jdk/pull/21041