[fortran, patch] Handle POW/CPOW builtins for all floating-point types

[FX]

Committed as rev. 163721, thanks for the review.

Regarding array descriptors:

> "Array intrinsics *do not work at all*. This is a limitation of the array descriptor structure, which I cannot overcome by myself."
> Wouldn't it work to use a patch as the following - or have I missed some fine print? I think we have enough bits to fit 8 DTYPEs - and without the following patch I only count 7.

I'll have to check... in your idea, would a real(kind=16) descriptor have only GFC_DTYPE_FLOAT128 set, or (GFC_DTYPE_FLOAT128 | GFC_DTYPE_REAL)?

FX