RFR: 8303002: Reject packed structs from linker [v6]

[Jorn Vernee]

> This patch adds checks in AbstractLinker to reject packed structs and structs with excess padding (e.g. unnamed bitfields), since both of those are currently not supported, and the ABI/spec seems too vague to base support on.

Jorn Vernee has updated the pull request incrementally with four additional commits since the last revision:

 - restrictions -> characteristics
 - polish pt. 2
 - polish wording
 - review comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13164/files
  - new: https://git.openjdk.org/jdk/pull/13164/files/eaf0c9c6..62cd5ee4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13164&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13164&range=04-05

  Stats: 16 lines in 2 files changed: 1 ins; 2 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/13164.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13164/head:pull/13164

PR: https://git.openjdk.org/jdk/pull/13164