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: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>