Changing SnoopMask type from uint64_t to std::bitset
This allows the size of the system (core count) to be larger than the current system.

The # of slave ports was 2xcore + 1 for a basic example/se.py simulation with --caches option.

Thus the maximum core size when using SnoopFilter was 31 due to the 64 bit type.

Bitset allows this to be expanded, allowing the use of SnoopFilter to be used in larger core count systems.
TODO: Dynamically allow the build system to specify the MAX_CORES macro value.

Testing done on config file example/se.py with --caches option.

(Modified code to add the SnoopFilter() into the SystemXBar)