RFR: 8303002: Reject packed structs from linker [v3]

[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 one additional commit since the last revision:

  add javadoc + fixes for trailing padding

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13164/files
  - new: https://git.openjdk.org/jdk/pull/13164/files/e11fc6b5..0698b9e3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13164&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13164&range=01-02

  Stats: 57 lines in 3 files changed: 30 ins; 2 del; 25 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