target/ppc: Introduce gen_icount_io_start

target/ppc: Introduce gen_icount_io_start

Create a function to handle the details for interacting with icount.

Force the exit from the tb via DISAS_TOO_MANY, which allows chaining
to the next tb, where the code emitted for gen_tb_start() will
determine if we must exit.  We can thus remove any matching
conditional call to gen_stop_exception.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20210517205025.3777947-2-matheus.ferst@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>