Add Port::fequal for longdouble comparisons.

This makes me a little nervous inside to change, as it would be useful to have access to ```realsize``` and ```realpad``` from port for the sake of portability, but this is not possible due to the ties target has with the front-end.

Maybe should change real comparisons to:
``` return abs(x, y) <= MAX(LDBL_EPSILON, LDBL_EPSILON * MAX(abs(x), abs(y)));```

...