RFA: Add implementation of cabsl()

[Nicholas Clifton]

Hi Corinna,

> This looks like an excellent start into the wonderful world of long
> double function support.  Yes, please apply.

Thanks - done.

> So, if I want to export the functions from Cygwin, which new functions
> are now supported?  AFAICs, the list is
>
>    cabsl, cimagl, creall, finitel, hypotl, sqrtl
>
> That's it?

Yup, that's it.  For now ...

Cheers
   Nick