Tweaks to dunnington configuration (x86_64/core2).
Tweaks to dunnington configuration (x86_64/core2).

Details:
- Updated BLIS_DEFAULT_KC_D from 256 to 384.
- Enabled cache blocksize extension of up to 25% for MC and KC (for
  double-precision real).

scibuilder/blis,arm-hpc/blis,xianyi/blis,xianyi/blis,scibuilder/blis,arm-hpc/blis,xianyi/blis,rwl/blis,arm-hpc/blis,xianyi/blis,audiofilter/blis,audiofilter/blis,rwl/blis,arm-hpc/blis,rwl/blis,scibuilder/blis,audiofilter/blis,scibuilder/blis,rwl/blis,audiofilter/blis