If you want compatibility between x86 CPUs and ARM/PowerPC/MIPS then you have to not use the extended 80-bit results. The 80-bit results can be more accurate (although not always, it depends on exactly how they are used) but the OP wasn't concerned with maximum accuracy but with exact repeatability.
Similarly, if you want cross-architecture consistency you need to not use fmadd instructions because fmadd gives different results from separate multiply and add. The fmadd results are more accurate, but that is not the point in this case.
Similarly, if you want cross-architecture consistency you need to not use fmadd instructions because fmadd gives different results from separate multiply and add. The fmadd results are more accurate, but that is not the point in this case.