Remove constructors from longdouble and IN_GCC code from frontend.

Clears up a lot of the ifdef code in the D front-end that is specific to gdc-only.  Allows longdouble to be used in a union, and add more capabilities on the operations supported by the glue (whilst still not allowing conversions from longdouble -> float/double.

Needs review, as might be relevant to spot any potential cleanups from what's been added/removed.