Tidy up FP emulation in C compiler with nice front-end routines like fp_long_to_doubl...

Tidy up FP emulation in C compiler with nice front-end routines like fp_long_to_double() rather than register-level FP-11 emulation, although the register-level emulation is still used to some extent in the FP backend, also change the definition of _FLOAT and _DOUBLE in libcross, although it isn't used in compiler yet