target-ppc, tcg: fix usermode segfault with pthread_create()

Programs run under qemu-ppc64 on an x86_64 host currently segfault
if they use pthread_create() due to the adjustment made to the NIP in
commit bd6fefe7.

This patch changes cpu_loop() to set the NIP back to the
pre-incremented value before calling do_syscall(), which causes the
correct address to be used for the new thread and corrects the fault.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>