perf tools: Add helper functions for some sort keys

perf tools: Add helper functions for some sort keys

The 'trace', 'srcline' and 'srcfile' sort keys updates hist entry's
field later.  With the hierarchy mode, those fields are passed to a
matching entry so it needs to identify the sort keys.

Signed-off-by: Namhyung Kim <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>