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: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NAlexander Graf <agraf@suse.de>

target-s390x/translate.c

@@ -320,7 +320,7 @@ static void gen_program_exception(DisasContext *s, int code)
 
 static inline void gen_illegal_opcode(DisasContext *s)
 {
-    gen_program_exception(s, PGM_SPECIFICATION);
+    gen_program_exception(s, PGM_OPERATION);
 }
 
 #ifndef CONFIG_USER_ONLY