[jdk21u-dev] RFR: 8320370: NMT: Change MallocMemorySnapshot to simplify code.

[Aleksey Shipilev]

Clean backport to improve NMT initialization and provide cleanliness for other backports.

Additional testing:
 - [x] MacOS AArch64 server fastdebug, `runtime/NMT`

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Commit messages:
 - Backport a7f60164063bdf95437326e7550fd65ff91b5566

Changes: https://git.openjdk.org/jdk21u-dev/pull/552/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=552&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320370
  Stats: 19 lines in 5 files changed: 0 ins; 12 del; 7 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/552.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/552/head:pull/552

PR: https://git.openjdk.org/jdk21u-dev/pull/552