This patch adds a global-global (gAG) branch predictor, used by specifying the "global" predType.
From BranchPredictor, it uses the following variables for configuration: 
 - globalPredictorSize
 - globalCtrBits
 - globalHistoryBits

As such, it may conflict with the ongoing effort to factor out the redundant variable in the predictors.

Prior to submission, this was tested by building build/ARM/gem5.debug, and run against a subset of the SPEC CPU2006 benchmarks. Statistics generated seem valid, and it caused no execution errors.