powerpc32: Fix timebase synchronization on 32-bit powermacs

The variable `timebase' used to transfer the current timebase value
from one cpu to the other in smp_core99_give/take_timebase was only
an unsigned long, i.e. 32 bits on 32-bit machines.  It needs to be
64 bits.  This makes it a u64, and fixes the issue reported by Kyle
Moffett, that the two cpus see wildly different values for the time
of day.

Signed-off-by: Paul Mackerras <paulus@samba.org>