Sandy Bridge configuration, micro-kernel update.

Details:
- Minor updates to bli_config and bli_kernel.h for sandybridge
  configuration.
- Renamed existing AVX intrinsic-based micro-kernel file to
  bli_gemm_int_d8x4.c.
- Added new file, bli_gemm_asm_d8x4.c, which provides assembly-based
  gemm micro-kernels for single- and double-precision real.