target/ppc: set is_jmp on ppc_tr_breakpoint_check

The use of GDB breakpoints was broken by b0c2d521 ("target/ppc: convert
to TranslatorOps", 2018-02-16).

Fix it by setting is_jmp, so that we break from the translation loop
as originally intended.
Tested-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reported-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

target/ppc/translate.c

@@ -7482,6 +7482,7 @@ static bool ppc_tr_breakpoint_check(DisasContextBase *dcbase, CPUState *cs,
     DisasContext *ctx = container_of(dcbase, DisasContext, base);
 
     gen_debug_exception(ctx);
+    dcbase->is_jmp = DISAS_NORETURN;
     /* The address covered by the breakpoint must be included in
        [tb->pc, tb->pc + tb->size) in order to for it to be
        properly cleared -- thus we increment the PC here so that