add openat64 on linux

Author: Mic92

src/unix/notbsd/mod.rs

@@ -917,6 +917,9 @@ extern {
                   offset: off64_t)
                   -> *mut ::c_void;
     pub fn open64(path: *const c_char, oflag: ::c_int, ...) -> ::c_int;
+    pub fn openat64(fd: ::c_int,
+                    path: *const c_char,
+                    oflag: ::c_int, ...) -> ::c_int;
     pub fn pread64(fd: ::c_int, buf: *mut ::c_void, count: ::size_t,
                    offset: off64_t) -> ::ssize_t;
     pub fn pwrite64(fd: ::c_int, buf: *const ::c_void, count: ::size_t,