Fix error codes in read and sched_yield.

sysdeps/nacl/sched_yield.c

Index: sysdeps/nacl/sched_yield.c
diff --git a/sysdeps/nacl/sched_yield.c b/sysdeps/nacl/sched_yield.c
index 0bbd754965624959ef3c6ed7e1f77bd4efba25ce..38a03e4b273a9dbd939d3296c582cd99e0781189 100644
--- a/sysdeps/nacl/sched_yield.c
+++ b/sysdeps/nacl/sched_yield.c
@@ -9,10 +9,10 @@ int __sched_yield (void)
 {
   int result = __nacl_irt_sched_yield ();
   if (result != 0) {
-    errno = -result;
+    errno = result;
     return -1;
   }
-  return -result;
+  return 0;
 }
 libc_hidden_def (__sched_yield)
 weak_alias (__sched_yield, sched_yield)