Move away from the __ARCH_WANT_SYSCALL_NO_AT system calls.

Modern architectures only get the *at(2) system calls. For example,
aarch64 doesn't have open(2), and expects userspace to use openat(2)
instead.

Change-Id: I87b4ed79790cb8a80844f5544ac1a13fda26c7b5
diff --git a/libc/include/stdio.h b/libc/include/stdio.h
index a13cfea..409afea 100644
--- a/libc/include/stdio.h
+++ b/libc/include/stdio.h
@@ -260,7 +260,8 @@
 		__printflike(2, 0);
 #endif
 
-int	 rename (const char *, const char *);
+extern int rename(const char*, const char*);
+extern int renameat(int, const char*, int, const char*);
 
 int	 fgetpos(FILE * __restrict, fpos_t * __restrict);
 int	 fsetpos(FILE *, const fpos_t *);