Implement fast il wrapper for v4 Monitor.Enter.

Implement fast il wrapper for v4 Monitor.Enter.

	* monitor.c (mono_monitor_get_fast_enter_method): Add support
	for v4 Enter.

	* monitor.c (mono_monitor_is_il_fastpath_wrapper): New function
	that returns true for any of the monitor il wrappers.

	* runtime.c (mono_runtime_is_critical_method): New function
	that reports if a given method is critical and must not be interrupted.

	* mono-threads.c (is_thread_in_critical_region): As the runtime if
	the top method is critical in addition to the GC.

	* method-to-ir.c (mini_emit_inst_for_method): Enable the IL fastpath
	for v4 Monitor.Enter.