GEMM: skylake: improve the performance when m is small

optimize on sgemv_n for small n

Move x86_64 DOMATCOPY_RT back to the C implementation

Initial code for Cooperlake BF16 GEMM kernel

Improve the performance of zasum and casum with AVX512 intrinsic

Improve the performance of dasum and sasum when SMP is defined

Improve the performance of rot by using AVX512 and AVX2 intrinsic

1. Add a new API -- sbgemv to support bfloat16 based gemv
2. Implement a generic kernel for sbgemv
3. Implement an avx512-bf16 based kernel for sbgemv

Signed-off-by: Chen, Guobing <guobing.chen@intel.com>

dgemm_tcopy_16_skylakex.c CNAME function should return an int, add a
return 0 similar to other files.

As observed using GCC 10 using -march=native -ftree-vectorize
on Knights Landing, it is now smart enough to find clobbers inside
non-inlined static functions.

In particular, sgemv counted on a kernel to preserve the whole
%ymm2 register (since it was not in the clobber list), but the top
part was destroyed by vzeroupper. This caused many tests to fail.

This patch makes sure all xmm (and ymm/zmm by extension) registers
are listed as clobbered to avoid this happening, as most kernels
already did correctly in fact.

The
`#if defined(SKYLAKEX) || defined (COOPERLAKE)`
from that commit was before #include "common.h" so caused the
compiled function to be empty, returning garbage results for
qualifying sgemm's on those architectures.

Closes #2914

Revert special handling of Windows xNRM2 and enable C+intrinsics kern…

Rename "HALF" precision functions (sh prefix) to "BFLOAT16" with "sb" prefix

add fninit to reset fpu registers before assembler routines