perf tools: Add perf_exe() helper to find perf binary

Also convert one existing user.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20190224153722.27020-9-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/util.c

@@ -568,3 +568,13 @@ out:
 
 	return tip;
 }
+
+char *perf_exe(char *buf, int len)
+{
+	int n = readlink("/proc/self/exe", buf, len);
+	if (n > 0) {
+		buf[n] = 0;
+		return buf;
+	}
+	return strcpy(buf, "perf");
+}