perf mem: Enable sampling loads and stores simultaneously

This patch modifies perf mem to default to sampling loads and stores
simultaneously. It could only do one or the other before yet there was
no hardware restriction preventing simultaneous collection. With this
patch, one run is sufficient to collect both.

It is still possible to sample only loads or stores by using the
-t option:
 $ perf mem -t load rec
 $ perf mem -t load rep
Or
 $ perf mem -t store rec
 $ perf mem -t store rep

The perf report TUI will show one event at a time. The store output will
contain a Weight column which will be empty.

In V2, we updated the man pages to reflect the change and also simplify
the initialization of the argv vector passed to the cmd_*() functions as
per LKML feedback.

In V3, we fixed typos in the changelog.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Joe Mario <jmario@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Fowles <rfowles@redhat.com>
Link: http://lkml.kernel.org/r/20141217152355.GA10053@thinkpad


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