Create a AVX512 enabled version of DGEMM

This patch adds dgemm_kernel_4x8_skylakex.c which is
* dgemm_kernel_4x8_haswell.s converted to C + intrinsics
* 8x8 support added
* 8x8 kernel implemented using AVX512

Performance is a work in progress, but already shows a 10% - 20%
increase for a wide range of matrix sizes.

kernel/x86_64/KERNEL.SKYLAKEX

@@ -2,18 +2,12 @@ include $(KERNELDIR)/KERNEL.HASWELL
 
 SGEMMKERNEL    =  sgemm_kernel_16x4_skylakex.S
 
+DGEMMKERNEL    =  dgemm_kernel_4x8_skylakex.c
 
-#DTRMMKERNEL    =  ../generic/trmmkernel_16x2.c
-#DGEMMKERNEL    =  dgemm_kernel_16x2_skylakex.S
-#DGEMMINCOPY    =  ../generic/gemm_ncopy_16.c
-#DGEMMITCOPY    =  ../generic/gemm_tcopy_16.c
-#DGEMMONCOPY    =  ../generic/gemm_ncopy_2.c
-#DGEMMOTCOPY    =  ../generic/gemm_tcopy_2.c
-#DGEMMINCOPYOBJ =  dgemm_incopy$(TSUFFIX).$(SUFFIX)
-#DGEMMITCOPYOBJ =  dgemm_itcopy$(TSUFFIX).$(SUFFIX)
-#DGEMMONCOPYOBJ =  dgemm_oncopy$(TSUFFIX).$(SUFFIX)
-#DGEMMOTCOPYOBJ =  dgemm_otcopy$(TSUFFIX).$(SUFFIX)
-
+DGEMMINCOPY    =  ../generic/gemm_ncopy_8.c
+DGEMMITCOPY    =  ../generic/gemm_tcopy_8.c
+DGEMMONCOPY    =  ../generic/gemm_ncopy_8.c
+DGEMMOTCOPY    =  ../generic/gemm_tcopy_8.c
 
 SGEMM_BETA = ../generic/gemm_beta.c
 DGEMM_BETA = ../generic/gemm_beta.c