The fix for 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 restore the alignment. Releases able to use C++11 can use `alignas(16)` to achieve this. Earlier releases can add a dummy 8-byte field for padding.

This is not generally a problem as since JDK 19 (JDK-8277822) the only use of `GuardedMemory` is for -Xcheck:jni, but Java arrays only have 8-byte alignment to begin with so are unaffected in normal use.

Prior to JDK 19 VM code only used GuardedMemory in debug builds, and no such usage had such a strict alignment requirement since some ZGC code in JDK 17 (it used movaps to process data).