We can't read env->xer directly, as it does not contain some bits of
XER. Instead, we should have a callback that uses cpu_read_xer to read
the complete register.

Fixes: da91a00f ("target-ppc: Split out SO, OV, CA fields from XER")
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20211014223234.127012-5-matheus.ferst@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>