RFR: 8302615: make JVMTI thread cpu time functions optional for virtual threads

[Serguei Spitsyn]

This is a minor JVM TI spec update for two timer functions `GetCurrentThreadCpuTime` and `GetThreadCpuTime` to allow implementations to support virtual threads.

The CSR is:
[JDK-8302616](https://bugs.openjdk.org/browse/JDK-8302616): make JVMTI thread cpu time functions optional for virtual threads

No testing is needed.

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Commit messages:
 - 8302615: make JVMTI thread cpu time functions optional for virtual threads

Changes: https://git.openjdk.org/jdk/pull/12604/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12604&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302615
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12604.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12604/head:pull/12604

PR: https://git.openjdk.org/jdk/pull/12604