Build without -fsingle-precision-constant (Dingux, GCW Zero)

In src/m68k/uae/fpp.cpp, floating-point constant generators use values
between 1.0e64 and 1.0e256, but those were being interpreted as if
they were constants of type float (1.0e64f .. 1.0e256f) instead of
constants of type double, as written in the code.