RFR: 8329109: Threads::print_on() tries to print CPU time for terminated GC threads

[Richard Reingruber]

Make use of `Threads::threads_do()` if at a safepoint to print the threads.
`Threads::threads_do()` uses thread list iterators that don't visit terminated threads.
If a non-java thread is about to terminate, it waits for in progress iterations (see [NonJavaThread::remove_from_the_list](https://github.com/openjdk/jdk/blob/47f33a59eaaffc74881fcc9e29d13ff9b2538c2a/src/hotspot/share/runtime/nonJavaThread.cpp#L97-L102)).

Only debug code calls `Threads::print_on()` outside a safepoint if I'm not mistaken so this solution should work in the relevant cases (jcmd, SIGQUIT, ...).

Testing:

The output of a minimal example with G1 is almost identical. The order of `NonJavaThreads` can differ.

More tests (besides GHA) are pending.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/18518/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18518&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329109
  Stats: 69 lines in 1 file changed: 43 ins; 14 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/18518.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18518/head:pull/18518

PR: https://git.openjdk.org/jdk/pull/18518