perf tools: Add a 'percore' event qualifier

Add a 'percore' event qualifier, like cpu/event=0,umask=0x3,percore=1/,
that sums up the event counts for both hardware threads in a core.

We can already do this with --per-core, but it's often useful to do
this together with other metrics that are collected per hardware thread.
So we need to support this per-core counting on a event level.

This can be implemented in only the user tool, no kernel support needed.

 v4:
 ---
 1. Add Arnaldo's patch which updates the documentation for
    this new qualifier.
 2. Rebase to latest perf/core branch

 v3:
 ---
 Simplify the code according to Jiri's comments.
 Before:
   "return term->val.percore ? true : false;"
 Now:
   "return term->val.percore;"

 v2:
 ---
 Change the qualifier name from 'coresum' to 'percore' according to
 comments from Jiri and Andi.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Tested-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1555077590-27664-2-git-send-email-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.h

@@ -50,6 +50,7 @@ enum term_type {
 	PERF_EVSEL__CONFIG_TERM_OVERWRITE,
 	PERF_EVSEL__CONFIG_TERM_DRV_CFG,
 	PERF_EVSEL__CONFIG_TERM_BRANCH,
+	PERF_EVSEL__CONFIG_TERM_PERCORE,
 };
 
 struct perf_evsel_config_term {
@@ -67,6 +68,7 @@ struct perf_evsel_config_term {
 		bool	overwrite;
 		char	*branch;
 		unsigned long max_events;
+		bool	percore;
 	} val;
 	bool weak;
 };
@@ -158,6 +160,7 @@ struct perf_evsel {
 	struct perf_evsel	**metric_events;
 	bool			collect_stat;
 	bool			weak_group;
+	bool			percore;
 	const char		*pmu_name;
 	struct {
 		perf_evsel__sb_cb_t	*cb;