[jdk21u-dev] RFR: 8315605: G1: Add number of nmethods in code roots scanning statistics

[Goetz Lindenmaier]

I backport this for parity with 21.0.4-oracle.

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Commit messages:
 - Backport 7ef059a75cdd2e96421a165ea1dd8391381bd7b5

Changes: https://git.openjdk.org/jdk21u-dev/pull/425/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=425&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/jdk21u-dev/pull/425.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/425/head:pull/425

PR: https://git.openjdk.org/jdk21u-dev/pull/425