perf hist: Implement output fields for mem stats

perf hist: Implement output fields for mem stats

This is a preparation for later changes to support mem_stat output.  The
new fields will need two lines for the header - the first line will show
type of mem stat and the second line will show the name of each item
which is returned by mem_stat_name().

Each element in the mem_stat array will be printed in percentage for the
hist_entry and their sum would be 100%.

Add new output field dimension only for SORT_MODE__MEM using mem_stat.

To handle possible name conflict with existing sort keys, move the order
of checking output field dimensions after the sort dimensions when it
looks for sort keys.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.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: Leo Yan <leo.yan@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lore.kernel.org/r/20250430205548.789750-7-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>