perf, tools, stat: Implement --metric-only mode

Add a new mode to only print metrics. Sometimes we don't care about
the raw values, just want the computed metrics. This allows more
compact printing, so with -I each sample is only a single line.

This also allows easier plotting and processing with other tools
because the output is normalized.

A few metrics are not supported and it does not support all
output modes (like -A and --per-thread), but works for all
the others.

To avoiding having to hardcode all the metrics in the code it uses
a two pass approach: first compute dummy metrics and only
print the headers in the print_metric callback. Then use the callback
to print the actual values. There are some additional changes
in the stat printout code to handle all metrics being on a single line.

Example:

% perf stat -a -I 1000 --metric-only
     1.001402499 frontend cycles idle     backend cycles idle      insn per cycle           stalled cycles per insn
     1.001402499  168.38%                 -                         0.35                    4.77
     2.002257235  183.18%                 -                         0.47                    3.64
     3.003652261  155.27%                 -                         0.38                    4.49

Signed-off-by: Andi Kleen <ak@linux.intel.com>