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

[Johan Sjölen]

> 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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16724/files
  - new: https://git.openjdk.org/jdk/pull/16724/files/b0c547b4..b209e881

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16724&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16724&range=02-03

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16724.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16724/head:pull/16724

PR: https://git.openjdk.org/jdk/pull/16724