RFR: 8364235: Fix for JDK-8361447 breaks the alignment requirements for GuardedMemory [v2]

[David Holmes]

> The fix for [JDK-8361447](https://bugs.openjdk.org/browse/JDK-8361447) added a new field to the `GuardHeader`, not realizing that the size of the `GuardHeader` must be such that the address of the user-data has the strictest necessary alignment (16-byte).
> 
> We need to add a padding field to restore the alignment.
> 
> A static assert is added to check the alignment.
> 
> Testing:
>  - tiers 1-3 (in progress)
> 
> Thanks

David Holmes has updated the pull request incrementally with one additional commit since the last revision:

  Correct comments as per reviewer comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26524/files
  - new: https://git.openjdk.org/jdk/pull/26524/files/a7502d8c..4d252e3c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26524&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26524&range=00-01

  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/26524.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26524/head:pull/26524

PR: https://git.openjdk.org/jdk/pull/26524