SE/FS: Define a const bool FullSystem which will equal FULL_SYSTEM.

SE/FS: Define a const bool FullSystem which will equal FULL_SYSTEM.

This constant will have the same value as FULL_SYSTEM but will not be usable
by the preprocessor. It can be substituted into places where FULL_SYSTEM is
used in a C++ context and will make it easier to find which parts of the
simulator still use FULL_SYSTEM with the preprocessor using grep.