RFR: 8349003: NativeCallStack::print_on() output is unreadable

[Zhengyu Gu]

Please review this trivial change that improves output readability.

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Commit messages:
 - 8349003: NativeCallStack::print_on() output is unreadable

Changes: https://git.openjdk.org/jdk/pull/23359/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23359&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349003
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23359.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23359/head:pull/23359

PR: https://git.openjdk.org/jdk/pull/23359