Make not_suspended parameter from ObjectMonitor::exit() have default value

The boolean parameter not_suspended is used to detect if we need to set the current JavaThread exiting the monitor as the previous owner (_previous_owner_tid). If not_suspended is true then we set _previous_owner_tid, otherwise we skip the write (modulo the JFR checks). This parameter is always true except when we call exit() from inside enter(). This happens when the JavaThread acquires the monitor but notices that it was suspended while being in the _thread_blocked state. Since in that case the JT was never really "the owner" we skip setting _previous_owner_tid.
This behavior of releasing the monitor is just an implementation detail of ObjectMonitor::enter() which doesn't need to be exposed in the exit() API. Therefore we can make not_suspended an optional parameter with a default value of 'true'. Then we would only need to explicitly specify it when calling exit() for the special case mentioned before.