cad/iverilog: fix build on GCC architectures

expression_evaluate.cc: In member function 'virtual bool ExpArithmetic::evaluate(Entity*, ScopeBase*, int64_t&) const':
expression_evaluate.cc:67: error: call of overloaded 'pow(int64_t&, int64_t&)' is ambiguous
/usr/include/math.h:257: note: candidates are: double pow(double, double)
/usr/include/c++/4.2/cmath:373: note: long double std::pow(long double, int)
/usr/include/c++/4.2/cmath:369: note: float std::pow(float, int)
/usr/include/c++/4.2/cmath:365: note: double std::pow(double, int)
/usr/include/c++/4.2/cmath:361: note: long double std::pow(long double, long double)
/usr/include/c++/4.2/cmath:357: note: float std::pow(float, float)