s390/tools: Use array instead of string initializer

The in-kernel disassembler intentionally uses nun-null terminated
strings in order to keep the arrays which contain mnemonics as small
as possible. GCC 15 however warns about this:

./arch/s390/include/generated/asm/dis-defs.h:1662:71: error: initializer-string
 for array of ‘char’ is too long [-Werror=unterminated-string-initialization]
 1662 |         [1261] = { .opfrag = 0xea, .format = INSTR_SS_L0RDRD, .name = "unpka" }, \

Get rid of this warning by using array initializers.

Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>