s390/tools: fix gcc 8 stringop-truncation warnings

Replace strncpy which has been used to copy a substring into buf
without NUL-termination with memcpy to avoid the following gcc 8
warnings:

arch/s390/tools/gen_opcode_table.c: In function ‘add_to_group.isra.4’:
arch/s390/tools/gen_opcode_table.c:260:2: warning: ‘strncpy’
output may be truncated copying 2 bytes from a string of length 19
[-Wstringop-truncation]
  strncpy(group->opcode, insn->opcode, 2);

In function ‘print_opcode_table’,
    inlined from ‘main’ at arch/s390/tools/gen_opcode_table.c:333:2:
arch/s390/tools/gen_opcode_table.c:286:4: warning: ‘strncpy’
output may be truncated copying 2 bytes from a string of length 19
[-Wstringop-truncation]
    strncpy(opcode, insn->opcode, 2);
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/tools/gen_opcode_table.c

@@ -257,7 +257,7 @@ static void add_to_group(struct gen_opcode *desc, struct insn *insn, int offset)
 	if (!desc->group)
 		exit(EXIT_FAILURE);
 	group = &desc->group[desc->nr_groups - 1];
-	strncpy(group->opcode, insn->opcode, 2);
+	memcpy(group->opcode, insn->opcode, 2);
 	group->type = insn->type;
 	group->offset = offset;
 	group->count = 1;
@@ -283,7 +283,7 @@ static void print_opcode_table(struct gen_opcode *desc)
 			continue;
 		add_to_group(desc, insn, offset);
 		if (strncmp(opcode, insn->opcode, 2)) {
-			strncpy(opcode, insn->opcode, 2);
+			memcpy(opcode, insn->opcode, 2);
 			printf("\t/* %.2s */ \\\n", opcode);
 		}
 		print_opcode(insn, offset);