* io/openat.c (__openat): Define instead of openat.
Use libc_hidden_def. Define openat with weak_alias.
* io/openat64.c (__openat64): Define instead of openat64.
Use libc_hidden_def. Define openat64 with weak_alias.
* sysdeps/unix/sysv/linux/openat.c: Likewise.
* libio/genops.c: Include <sched.h> for __sched_yield decl.