RFR: 8316098: Revise signature of numa_get_leaf_groups

[Albert Mingkun Yang]

Simple refactoring to better reflect NUMA node id is non-negative using unsigned type.

More cleanup can possibly be done to avoid the use of `checked_cast` in `os_windows.cpp`, but since the NUMA code in Windows is unreachable (JDK-8244065), I went to for the smallest diff there in order to avoid adding more untested code.

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Commit messages:
 - os-node-id-api

Changes: https://git.openjdk.org/jdk/pull/15786/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15786&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316098
  Stats: 11 lines in 7 files changed: 1 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15786.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15786/head:pull/15786

PR: https://git.openjdk.org/jdk/pull/15786