RFR: 8315605: G1: Add number of nmethods in code roots scanning statistics

[Thomas Schatzl]

Hi all,

  please review this change that adds nmethod count statistics during Code Root Scan to be able to implement [JDK-8030815](https://bugs.openjdk.org/browse/JDK-8030815).

Testing: gha, checking that the message appears manually

Thanks,
  Thomas

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Commit messages:
 - Report code roots scan time and count at the same time
 - add code root nmethod count

Changes: https://git.openjdk.org/jdk/pull/15610/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15610&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315605
  Stats: 39 lines in 3 files changed: 38 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15610.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15610/head:pull/15610

PR: https://git.openjdk.org/jdk/pull/15610