JVMTI implementation: native methods in agent must be available to Java code

Implement this part of the JVMTI spec --

    Libraries loaded with -agentlib: or -agentpath:
    will be searched for JNI native method implementations to facilitate the
    use of Java programming language code in tools, as is needed for
    bytecode instrumentation.

    The agent libraries will be searched after all other libraries have been
    searched ...

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