[POWERPC] Fix wraparound problem in smp-tbsync on 32-bit

The patch below fixes an arithmetic wrap-around issue on 32bit machines
using smp-tbsync. Without this patch a timebase value over
0x000000007fffffff will hang the boot process while bringing up
secondary CPUs.

Signed-off-by: Adrian Cox <adrian@humboldt.co.uk>
Signed-off-by: Paul Mackerras <paulus@samba.org>