[JVMCI] allow printing to tty from unattached libgraal thread

Currently, HotSpotJVMCIRuntime.writeDebugOutput does nothing if the current thread is not attached to HotSpot (i.e., Thread::current_or_null() == NULL). This means crucial debug info can be lost.

The only reason for the current behavior is that writeDebugOutput passes a Java byte array to C++ code and the C++ code calls back into Java to decode the byte array into a native array. These call backs require the current thread to be attached to the VM.

Moving the Java-to-native-buffer decoding into Java avoids the need for the call backs and hence the requirement for the current thread to be attached.