ruby: replace global g_system_ptr with per-object pointers

ruby: replace global g_system_ptr with per-object pointers

This is another step in the process of removing global variables
from Ruby to enable multiple RubySystem instances in a single simulation.

With possibly multiple RubySystem objects, we can no longer use a global
variable to find "the" RubySystem object.  Instead, each Ruby component
has to carry a pointer to the RubySystem object to which it belongs.