add sched_getcpu

This is a GNU extension, but a fairly minor one, for a system call that
otherwise has no libc wrapper.

include/sched.h

@@ -76,6 +76,7 @@ void free(void *);
 
 typedef struct cpu_set_t { unsigned long __bits[128/sizeof(long)]; } cpu_set_t;
 int __sched_cpucount(size_t, const cpu_set_t *);
+int sched_getcpu(void);
 int sched_getaffinity(pid_t, size_t, cpu_set_t *);
 int sched_setaffinity(pid_t, size_t, const cpu_set_t *);
 

src/sched/sched_getcpu.c

+#define _GNU_SOURCE
+#include <sched.h>
+#include "syscall.h"
+
+int sched_getcpu(void)
+{
+	int r;
+	unsigned cpu;
+
+	r = __syscall(SYS_getcpu, &cpu, 0, 0);
+	if (!r) return cpu;
+	return __syscall_ret(r);
+}