perf report: Fall-back to function name comparison for -g srcline

When a callchain entry has no srcline available, we ended up comparing
the instruction pointer. I consider this to be not too useful. Rather, I
think we should group the entries by function name, which this patch
adds. For people who want to split the data on the IP boundary, using
`-g address` is the correct choice.

Before:

~~~~~
   100.00%    38.86%  [.] main
            |
            |--61.14%--main inlining.cpp:14
            |          std::norm<double> complex:664
            |          std::_Norm_helper<true>::_S_do_it<double> complex:654
            |          std::abs<double> complex:597
            |          std::__complex_abs complex:589
            |          |
            |          |--56.03%--hypot
            |          |          |
            |          |          |--8.45%--__hypot_finite
            |          |          |
            |          |          |--7.62%--__hypot_finite
            |          |          |
            |          |          |--2.29%--__hypot_finite
            |          |          |
            |          |          |--2.24%--__hypot_finite
            |          |          |
            |          |          |--2.06%--__hypot_finite
            |          |          |
            |          |          |--1.81%--__hypot_finite
...
~~~~~

After:

~~~~~
   100.00%    38.86%  [.] main
            |
            |--61.14%--main inlining.cpp:14
            |          std::norm<double> complex:664
            |          std::_Norm_helper<true>::_S_do_it<double> complex:654
            |          std::abs<double> complex:597
            |          std::__complex_abs complex:589
            |          |
            |          |--60.29%--hypot
            |          |          |
            |          |           --56.03%--__hypot_finite
            |          |
            |           --0.85%--cabs
~~~~~

Signed-off-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yao Jin <yao.jin@linux.intel.com>
Link: http://lkml.kernel.org/r/20171009203310.17362-7-milian.wolff@kdab.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>