tools: bpftool: use PERF_SAMPLE_TIME instead of reading the clock

Ask the kernel to include sample time in each even instead of
reading the clock.  This is also more accurate because our
clock reading was done when user space would dump the buffer,
not when sample was produced.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>