perf tools: Add mode argument to sort_help()

Some sort keys are meaningful only in a specific mode - like branch
stack and memory (data-src).  Add the mode to skip unnecessary ones.
This will be used for 'perf mem report' later.

While at it, change the prefix for the -F/--fields option to remove
the duplicate part.

Before:

  $ perf report -F
   Error: switch `F' requires a value
   Usage: perf report [<options>]

      -F, --fields <key[,keys...]>
  			  output field(s): overhead period sample  overhead overhead_sys
  			  overhead_us overhead_guest_sys overhead_guest_us overhead_children
  			  sample period weight1 weight2 weight3 ins_lat retire_lat
  			  ...
After:

  $ perf report -F
   Error: switch `F' requires a value
   Usage: perf report [<options>]

      -F, --fields <key[,keys...]>
  			  output field(s): overhead overhead_sys overhead_us
  			  overhead_guest_sys overhead_guest_us overhead_children
  			  sample period weight1 weight2 weight3 ins_lat retire_lat
  			  ...

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20240731235505.710436-5-namhyung@kernel.org


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