perf annotate: Add symbol__calc_percent function

Add symbol__calc_percent function, that calculates annotation data for
symbol and put the data in the struct annotation_line::samples array.

Committer notes:

Made symbol__calc_percent non static to be used in the next two patches,
which will get some fixups from jolsa, doing it this way to keep this
bisectable.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20171011150158.11895-18-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>