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

[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

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Commit messages:
 - 8364235: Fix for JDK-8361447 breaks the alignment requirements for GuardedMemory

Changes: https://git.openjdk.org/jdk/pull/26524/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26524&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364235
  Stats: 11 lines in 1 file changed: 9 ins; 2 del; 0 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