target/ppc: fix single-step exception regression

target/ppc: fix single-step exception regression

Commit 6086c75 (target/ppc: Replace POWERPC_EXCP_BRANCH with
DISAS_NORETURN) broke the generation of exceptions when
CPU_SINGLE_STEP or CPU_BRANCH_STEP were set, due to nip always being
reset to the address of the current instruction.
This fix leaves nip untouched when generating the exception.

Signed-off-by: Luis Pires <luis.pires@eldorado.org.br>
Reported-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20210602125103.332793-1-luis.pires@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>