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.

The main target is with using --topdown, but it also works with
-T and standard perf stat. A few metrics are not supported.

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.

One issue is that the column code doesn't know in advance what events
are not supported by the CPU, and it would be hard to find out
as this could change based on dynamic conditions. That causes
empty columns in some cases.

The output can be fairly wide, often you may need more than 80 columns.

Example:

% perf stat -a -I 1000 --metric-only
     1.000604977 frontend cycles idle     backend cycles idle      insn per cycle           stalled cycles per insn  branch-misses of all branches
     1.000604977                                                    0.76                                             2.35%
     2.000924680                                                    0.72                                             2.34%
     3.001139592                                                    0.76                                             2.57%
     4.001358452                                                    0.73                                             2.44%

v2: Lots of updates.
v3: Use slightly narrower columns
Signed-off-by: Andi Kleen <ak@linux.intel.com>