Control coprocessor has the system library

	* src/vm/cp0.hh, src/vm/cp0.hxx:
	(set_system_library) New.
	(raise_syscall) Invoke system library if any.
	* src/vm/cpu.hh, src/vm/cpu.hxx:
	(set_system_library) Remove.
	* src/vm/cpu.cc:
	(syscall) Raise a syscall exception through cp0.
	* src/vm/virtual_machine.hh, src/vm/virtual_machine.hxx:
	(get_cp0) New non-const accessor.
	(set_system_library) Remove.
	* src/vm-tasks.cc:
	Access cp0 to set system library.