target/ppc: Introduce DISAS_{EXIT,CHAIN}{,_UPDATE}

Rewrite ppc_tr_tb_stop to handle these new codes.

Convert ctx->exception into these new codes at the end of
ppc_tr_translate_insn, prior to pushing the change back
throughout translate.c.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20210512185441.3619828-8-matheus.ferst@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>