S/390: Dump unknown instructions according to their length.

S/390: Dump unknown instructions according to their length.

Unknown instructions are currently just dumped as .long 1234.  On
S/390 we can do a bit better since the instruction length is encoded
in the opcode.  That way also unknown instructions can be skipped
according to their real length.  That way we can continue correctly
after that instruction.  However, there are also some drawbacks with
that behavior when dumping data.  So for now that behavior is only
enabled for text section but even there it might mess things up when
having a literal pool embedded in the code.  Therefore I've left the
feature disabled by default and have added the -Minsnlength option to
enable it explicitely.

opcodes/ChangeLog:

2016-06-10  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* s390-dis.c (option_use_insn_len_bits_p): New file scope
	variable.
	(init_disasm): Handle new command line option "insnlength".
	(print_s390_disassembler_options): Mention new option in help
	output.
	(print_insn_s390): Use the encoded insn length when dumping
	unknown instructions.