linux-user/ppc: Report correct fault address for data faults

For faults on loads and stores, ppc_cpu_handle_mmu_fault() in
target/ppc/user_only_helper.c stores the offending address
in env->spr[SPR_DAR]. Report this correctly to the guest
in si_addr, rather than incorrectly using the address of the
instruction that caused the fault.

This fixes the test case in
https://bugs.launchpad.net/qemu/+bug/1077116


for ppc, ppc64 and ppc64le.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>