Accept up to 13 ULP difference.
This fixes running "checkasm --test=ac3dsp 3044836819" on ARM.
Depending on how the SIMD implementations aggregate numbers,
larger/smaller values might not end up accumulated in exactly
the same way; the current NEON implementation for ARM aggregates
into vectors of 2 elements. If it would aggregate into vectors
of 4 elements instead, like the AArch64 version does, this particular
case would end up with a smaller difference.
8e4c904c8e