py: Add support for 64-bit NaN-boxing object model, on 32-bit machine.

To use, put the following in mpconfigport.h:

    #define MICROPY_OBJ_REPR (MICROPY_OBJ_REPR_D)
    #define MICROPY_FLOAT_IMPL (MICROPY_FLOAT_IMPL_DOUBLE)
    typedef int64_t mp_int_t;
    typedef uint64_t mp_uint_t;
    #define UINT_FMT "%llu"
    #define INT_FMT "%lld"

Currently does not work with native emitter enabled.