remove unused __getdents, rename and move file

the __-prefixed filename does not make sense when the only purpose of
this file is implementing a public function that's not used as a
backend for implementing the standard dirent functions.

src/dirent/__getdents.c → src/linux/getdents.c

@@ -1,12 +1,9 @@
 #include <dirent.h>
 #include "syscall.h"
-#include "libc.h"
 
-int __getdents(int fd, struct dirent *buf, size_t len)
+int getdents(int fd, struct dirent *buf, size_t len)
 {
 	return syscall(SYS_getdents, fd, buf, len);
 }
 
-weak_alias(__getdents, getdents);
-
 LFS64(getdents);