clang: Fix recently introduced clang compilation errors

This patch makes the code compile with clang 2.9 and 3.0 again by
making two very minor changes. Firt, it maintains a strict typing in
the forward declaration of the BaseCPUParams. Second, it adds a
FullSystemInt flag of the type unsigned int next to the boolean
FullSystem flag. The FullSystemInt variable can be used in
decode-statements (expands to switch statements) in the instruction
decoder.

SE/FS: Make SE vs. FS mode a runtime parameter.

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.