RFR: 8320370: NMT: Change MallocMemorySnapshot to simplify code. [v4]

[Gerard Ziemski]

On Thu, 7 Dec 2023 11:18:50 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Hi,
>> 
>> `MallocMemorySnapshot` used to be initialized in a quite non-standard way, using global placement new on an array of `size_t` of the size required to fit `MallocMemorySnapshot`. This looks like it was intended to circumvent some `ResourceObj` internals, but I'm unsure of its purpose. This change does what you expect a regular initialization of a global variable to look like.
>> 
>> ~Currently running through tier1.~ GHA passed and so did tier1 on Oracle CI.
>
> Johan Sjölen has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Didn't remove them
>  - Redo

Marked as reviewed by gziemski (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/16724#pullrequestreview-1770508882