RFR: 8342730: Get rid of SummaryDiff in VMATree

[Afshin Zafari]

`SummaryDiff` result of any `register_mapping()` call, is passed into the function rather than returned. This enables avoiding copying the structure to/from the stack.

Tests:
local linux-x64-debug: NMT gtests and JTREGs passed.

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Commit messages:
 - 8342730: Get rid of SummaryDiff in VMATree

Changes: https://git.openjdk.org/jdk/pull/26761/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26761&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342730
  Stats: 367 lines in 9 files changed: 99 ins; 3 del; 265 mod
  Patch: https://git.openjdk.org/jdk/pull/26761.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26761/head:pull/26761

PR: https://git.openjdk.org/jdk/pull/26761