Investigate perf impact of VirtualThread notifyJVMTI APIs

[fengxue-IS]

notifyJvmtiMount
notifyJvmtiUnmount

These calls are used in every VirtualThread transition, #17372 added FastJNI support to reduce the need of transition from JIT to VM. As these are only useful for synchronization when a JVMTI agent is attached, VM should detect if such path are actually used and avoid calling such if possible.

As a prototype, we should detect if JVMTI agent is attached, or allowed to attach during runtime (-XX:+EnableDynamicAgentLoading). if neither apply, then VM should skip the execution of these code and directly return.

[fengxue-IS]

Starting point:

struct J9JavaVM {
    ...
    BOOLEAN agentEnabled;
}

// during vm init:
if ("-XX:-EnableDynamicAgentLoading" && "no agent attached on startup") {
    vm->agentEnabled = false;
} else {
    vm->agentEnabled = true;
}

// javanextvmi
JVM_VirtualThreadMountBegin(...) { // same apply to unmount & end
    if (vm->agentEnabled) {
        // execute original work
    }
}

agentEnabled can also be implemented as a vm flag if available.
hideframes(...) will need to be verified if it have effect for non-jvmti related code, if it does, then that can not be skipped.

[thallium]

Performance improvement (on my xLinux fyre vm):

• Skynet: 1456.019 ±(99.9%) 46.318 ms/op -> 1402.605 ±(99.9%) 41.741 ms/op
• PingPong: 2745.477 ±(99.9%) 120.398 ms/op -> 2482.003 ±(99.9%) 52.346 ms/op