ruby: replace global g_abs_controls with per-RubySystem var

ruby: replace global g_abs_controls with per-RubySystem var

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

The list of abstract controllers is per-RubySystem and should be
represented that way, rather than as a global.

Since this is the last remaining Ruby global variable, the
src/mem/ruby/Common/Global.* files are also removed.