ppc64el: IEEE 128-bit long double instead of IBM 128-bit long double

For the opening of trixie, both GCC and LLVM are prepared to change the 
long double ABI from  IBM 128-bit long double to  IEEE 128-bit long 
double.  That is a change that other distros did long ago.

For reference, please see
https://developers.redhat.com/articles/2023/05/16/benefits-fedora-38-long-double-transition-ppc64le#

I'm proposing to do this "transition" inline, doing just no-change 
binNMUs in most cases.  long double in the ABI is not that common. 
Afaiu, Fedora and OpenSUSE did it this way, and also Ubuntu is doing it 
currently this way.  Ubuntu also has a (maybe incomplete) list of 
packages, which require binNMUs.

Matthias