s390x/GEMM: replace 0-init with peeled first iteration
s390x/GEMM: replace 0-init with peeled first iteration

... since it gains another ~2% of SGEMM and DGEMM performance on z15;
also, the code just called for that cleanup.

Signed-off-by: Marius Hillenbrand <fe1e41a2593d8a0d1128edc53cee7053b8cae934@linux.ibm.com>

xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS