perf trace: Support --events foo:bar --no-syscalls

I.e. support tracing just tracepoints, without strace like
raw_syscalls:*.

[acme@ssdandy linux]$ trace --no-sys --ev sched:*exec,sched:*switch,sched:*exit usleep 1
  0.048 (     ): sched:sched_process_exec:filename=/usr/bin/usleep pid=27298 old_pid=27298)
  0.369 (     ): sched:sched_switch:usleep:27298 [120] S ==> swapper/5:0 [120])
  0.452 (     ): sched:sched_process_exit:comm=usleep pid=27298 prio=120)
[acme@ssdandy linux]$

TODO: remove that (...) thing when --no-syscalls is specified.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-vn0hsixsbhm31b2rpj97r96k@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-trace.c

@@ -2625,7 +2625,8 @@ int cmd_trace(int argc, const char **argv, const char *prefix __maybe_unused)
 	if (trace.summary_only)
 		trace.summary = trace.summary_only;
 
-	if (!trace.trace_syscalls && !trace.trace_pgfaults) {
+	if (!trace.trace_syscalls && !trace.trace_pgfaults &&
+	    trace.evlist->nr_entries == 0 /* Was --events used? */) {
 		pr_err("Please specify something to trace.\n");
 		return -1;
 	}