[POWERPC] Fix problem with time not advancing on 32-bit platforms

[POWERPC] Fix problem with time not advancing on 32-bit platforms

This fixes a problem introduced in 5db9fa9593e2ff69f2b95f9d59229dc4faaa564d.
The last_jiffy per-cpu variable is only 32 bits on 32-bit machines, but it
was being compared with a 64-bit quantity (tb_next_jiffy), which resulted in
time not advancing.

This fixes it by changing last_jiffy to be 64 bits on all platforms.  With
this, we no longer need tb_last_stamp as a 32-bit version of tb_last_jiffy,
so this gets rid of tb_last_stamp and we just use tb_last_jiffy instead.
This also fixes a bug when the boot cpu is not online, because using
tb_last_stamp could have caused the wrong timebase origin value to be used
when calculating the time of day.

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