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

qemu-timer.h

@@ -98,9 +98,9 @@ static inline int64_t cpu_get_real_ticks(void)
 #else
     /* http://ozlabs.org/pipermail/linuxppc-dev/1999-October/003889.html */
     unsigned long junk;
-    __asm__ __volatile__ ("mftbu   %1\n\t"
-                          "mftb    %L0\n\t"
-                          "mftbu   %0\n\t"
+    __asm__ __volatile__ ("mfspr   %1,269\n\t"  /* mftbu */
+                          "mfspr   %L0,268\n\t" /* mftb */
+                          "mfspr   %0,269\n\t"  /* mftbu */
                           "cmpw    %0,%1\n\t"
                           "bne     $-16"
                           : "=r" (retval), "=r" (junk));