Don't compute global system uptime when reading CPU stats

Don't compute global system uptime when reading CPU stats

Don't compute "global system uptime" (i.e. total number of jiffies spent
by all processors) in read_stat_cpu() function so that we don't have to
pass the result from one function to another.
Global system uptime is used only when displaying CPU statistics, so
this is now done locally in the functions displaying CPU statistics.
Also as a consequence, field uptime has been removed from the
record_header structure associated with every sample statistics in
sar/sadc.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>