target-s390x: fix exception for invalid operation code

When an operation code is not recognized (ie invalid instruction) an
operation exception should be generated instead of a specification
exception. The latter is for valid opcode, with invalid operands or
modifiers.

This give a very basic GDB support in the guest, as it uses the invalid
opcode 0x0001 to generate a trap.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
