ext: McPAT interface changes and fixes
This patch includes software engineering changes and some generic bug fixes
Joel Hestness and Yasuko Eckert made to McPAT 0.8. There are still known
issues/concernts we did not have a chance to address in this patch.

High-level changes in this patch include:
1) Making XML parsing modular and hierarchical:
- Shift parsing responsibility into the components
- Read XML in a (mostly) context-free recursive manner so that McPAT input
files can contain arbitrary component hierarchies
2) Making power, energy, and area calculations a hierarchical and recursive
process
- Components track their subcomponents and recursively call compute
functions in stages
- Make C++ object hierarchy reflect inheritance of classes of components
with similar structures
- Simplify computeArea() and computeEnergy() functions to eliminate
successive calls to calculate separate TDP vs. runtime energy
- Remove Processor component (now unnecessary) and introduce a more abstract
System component
3) Standardizing McPAT output across all components
- Use a single, common data structure for storing and printing McPAT output
- Recursively call print functions through component hierarchy
4) For caches, allow splitting data array and tag array reads and writes for
better accuracy
5) Improving the usability of CACTI by printing more helpful warning and error
messages
6) Minor: Impose more rigorous code style for clarity (more work still to be
done)
Overall, these changes greatly reduce the amount of replicated code, and they
improve McPAT runtime and decrease memory footprint.

ext: add McPAT source

this patch adds the source for mcpat, a power, area, and timing modeling
framework.