perf hist: Add struct he_mem_stat

perf hist: Add struct he_mem_stat

The 'struct he_mem_stat' is to save detailed information about memory
instruction.  It'll be used to show breakdown of various data from
PERF_SAMPLE_DATA_SRC.  Note that this structure is generic and the
contents will be different depending on actual data it'll use later.

The information about the actual data will be saved in 'struct hists'
and its length is in nr_mem_stats.  This commit just adds ground works
and does nothing since hists->nr_mem_stats is 0 for now.

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/20250430205548.789750-5-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>