perf mem: Introduce weak function perf_mem_events__ptr()

Different architectures might use different event or different event
parameters for memory profiling, this patch introduces a weak
perf_mem_events__ptr() function which allows to return back a
architecture specific memory event.

Since the variable 'perf_mem_events' can be only accessed by the
perf_mem_events__ptr() function, mark the variable as 'static', this
allows the architectures to define its own memory event array.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/r/20201106094853.21082-3-leo.yan@linaro.org


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