AVX512 DGEMM TCOPY_16 Function

Optimize AVX512 parallel DGEMM performance

Something in the plain C parts of x86_64 cscal.c and zscal.c appears to be miscompiled by both gfortran9 and ifort when compiling for skylakex-avx512, even when the optimized Haswell microkernel is not in use.

As discussed on the original PR #2329, the "Apple Clang 11.0.3" that appears to be based the same LLVM release produces the same miscompilation of this file.

should have no performance impact on reasonably modern cpus and fixes occasional crashes in actual user code.

Fix avx512 sgemm performance bug when ldc is a multiple of 1024

Temporary fix for SKX STRSM

AVX2 STRSM kernels

Adjust SkylakeX GEMM3M parameters, add an AVX512 STRMM kernel and fix performance bugs in AVX2 s/c/z GEMM

The was a typo in iamax_sse.S where one of the comparison
was cmpeqps instead of cmpeqss. That misdetected index
for sequences where the minimum value was 0.

The leaq instructions in dscal_kernel_inc_8 modify x and x1 so they
must be declared as input/output constraints, otherwise the compiler
may assume the corresponding registers are not modified.

BTW some kernels were adjusted to improve performance