add sched_getcpu

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

src/sched/sched_getcpu.c

@@ -0,0 +1,13 @@
+#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);
+}