In a context of a native JVMCI compiler we need an access to JNIEnv of the compiler thread to be able to define a class for JVMCI compiler MXBean and register the MXBean into MBeanServer.

The proposed solution adds a HotSpotJVMCIRuntime.getCurrentJavaThread() method which returns the address of the HotSpot JavaThread C++ object for the current thread or 0L if called from an unattached JVMCI shared library thread.