perf tools: Work around lack of sched_getcpu in glibc < 2.6.

This patch fixes this build error with glibc < 2.6.

  CC       util/cloexec.o
cc1: warnings being treated as errors
util/cloexec.c: In function ‘perf_flag_probe’:
util/cloexec.c:24: error: implicit declaration of function
‘sched_getcpu’
util/cloexec.c:24: error: nested extern declaration of ‘sched_getcpu’
make: *** [util/cloexec.o] Error 1

Signed-off-by: Vinson Lee <vlee@twitter.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: stable@vger.kernel.org # 3.18+
Link: http://lkml.kernel.org/r/1427137761-16119-1-git-send-email-vlee@twopensource.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/cloexec.c

@@ -7,6 +7,12 @@
 
 static unsigned long flag = PERF_FLAG_FD_CLOEXEC;
 
+int __weak sched_getcpu(void)
+{
+	errno = ENOSYS;
+	return -1;
+}
+
 static int perf_flag_probe(void)
 {
 	/* use 'safest' configuration as used in perf_evsel__fallback() */

tools/perf/util/cloexec.h

@@ -3,4 +3,10 @@
 
 unsigned long perf_event_open_cloexec_flag(void);
 
+#ifdef __GLIBC_PREREQ
+#if !__GLIBC_PREREQ(2, 6)
+extern int sched_getcpu(void) __THROW;
+#endif
+#endif
+
 #endif /* __PERF_CLOEXEC_H */