RFR: JDK-8313656: assert(!JvmtiExport::can_support_virtual_threads()) with -XX:-DoJVMTIVirtualThreadTransitions

[Alex Menkov]

The change fixes several issues with capability management:
- handling can_support_virtual_threads capability. JvmtiExport::can_support_virtual_threads() should be set to true if we have one or more agent with can_support_virtual_threads capability;
- JvmtiManageCapabilities (used by GetPotentialCapabilities/AddCapabilities/RelinquishCapabilities/GetCapabilities) is not thread safe;
- SuspendAllVirtualThreads and ResumeAllVirtualThreads should check capability on the caller agent (and not JvmtiExport::can_support_virtual_threads()).

Testing: tier1..5

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Commit messages:
 - capabilities for VT

Changes: https://git.openjdk.org/jdk/pull/15219/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15219&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313656
  Stats: 53 lines in 3 files changed: 47 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/15219.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15219/head:pull/15219

PR: https://git.openjdk.org/jdk/pull/15219