target/ppc: Simplify gen_debug_exception

Two of the call sites that use gen_debug_exception have already
updated NIP.  Only ppc_tr_breakpoint_check requires the update.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20210512185441.3619828-7-matheus.ferst@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>