Make cpu_get_real_ticks use mfspr

PowerPC CPUs have had two ways to read the time base for quite some time now.
They provide it using the mfspr instruction or - if a special bit is set in
that opcode - using mftb. For timekeeping we're currently using mftb.

While trying to get Qemu up and running on an e500v2 system, I stumbled over
the CPU not supporting mftbu. It just throws an illegal instruction trap.

So let's read the SPR values instead. All PPC CPUs should support them anyways.

I tested this patch on an e500v2 system where it makes qemu work and on my 970MP
system with 32-bit user space where everything still works with this patch
applied.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: malc <av1474@comtv.ru>
