Physical layout is Qualcomms job, as Cortex-A57 is delivered as synthesizable IP. In addition Samsung had no issues with A57 on 20nm. In conclusion it is all Qualcomms fault.
ARM, in partnership with third parties and the fabs, also sells pre-made physical macros, which the smaller SoC makers have been using for years.
Recently they've been promoting that they've stepped up their efforts to make these macros more competitive with the custom work from top-tier SoC makers but they're probably not there yet, particularly not with the 20nm TSMC part and not in the timeframe Qualcomm needed it.
But I wouldn't discount the possibility that Qualcomm did in fact use a sub-par ARM physical macro as opposed to using a rushed internally designed layout. There are rumors that Qualcomm outsourced much of the CPU design in S810 which could have amounted to this.