[jdk26u] RFR: 8373625: CPUTimeCounters creates a total counter for unsupported GCs

[jyma22]

`CPUTimeCounters` unconditionally creates `CPUTimeGroups::CPUTimeType::gc_total`. Since only Parallel and G1 are supported by this framework/class, this leads to publishing a counter that always resolves to 0. This may be contradictory for an end-user especially so as we now support logging GC CPU time for any GC inside Hotspot. For an example using `-XX:+UseZGC -Xlog:cpu` we get

[7.907s][info][cpu] === CPU time Statistics =============================================================
[7.907s][info][cpu]                                                                             CPUs
[7.907s][info][cpu]                                                                s       %  utilized
[7.907s][info][cpu]    Process
[7.907s][info][cpu]      Total                                              186.9562  100.00      23.6
[7.907s][info][cpu]      Garbage Collection                                   0.6700    0.36       0.1
[7.907s][info][cpu]        GC Threads                                         0.6692    0.36       0.1
[7.907s][info][cpu]        VM Thread                                          0.0008    0.00       0.0
[7.907s][info][cpu] =====================================================================================

But `jcmd $(pgrep -n java) PerfCounter.print | grep -E "sun.threads.total_gc_cpu_time"` prints: `sun.threads.total_gc_cpu_time=0`.

Testing: passed tier1-4

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Commit messages:
 - Backport 30be94086aad42b99a15a05fe5115f552e8efb8b

Changes: https://git.openjdk.org/jdk26u/pull/5/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk26u&pr=5&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373625
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk26u/pull/5.diff
  Fetch: git fetch https://git.openjdk.org/jdk26u.git pull/5/head:pull/5

PR: https://git.openjdk.org/jdk26u/pull/5