Replaced vzeroall instruction with vxorpd and vmovapd for dgemm kernels
-both AVX2 and AVX512. vzeroall is expensive instruction and replaced it
with faster version of zeroing all registers. vzeroupper() instruction is
also added at the end of AVX2 kernels to avoid any AVX2/SSE transition
penalities. Kindly note only the main kernels are modified.
Change-Id: Ieb9bc629db01f0f94dd0e8e55550940d3d7eb2a4