scons: Enable stdc constant macros in stdint

Some versions of the standard libraries requires the define
__STD_CONSTANT_MACROS to enable the UINTxx_C and INTxx_C macros when
compiling C++ code. This changeset adds that define to the
preprocessor flags.
base: Split the SoftFloat wrapper simplify float(32|64) support

Split the SoftFloat80 class into a templatized GenericSoftFloat class
and a separate SoftFloat80Impl class. The generic class is intended to
provide a unified interface for all SoftFloat types. Using the generic
class, the amount of code duplication (particularly in overloaded
operators) can be somewhat reduced.