Rollup merge of #22862 - vhbit:broken-open, r=alexcrichton

 According to Apple's [arm64 calling convention](https://developer.apple.com/library/ios/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html#//apple_ref/doc/uid/TP40013702-SW1) varargs always are passed
through stack. Since `open` is actually a vararg function on Darwin,
it means that older declaration caused permissions to be taken from
stack, while passed through register => it set file permissions
to garbage and it was simply impossible to read/delete files after they
were created.

They way this commit handles it is to preserve compatibility with
existing code - it simply creates a shim unsafe function so all existing
callers continue work as nothing happened.

Author: Manishearth

src/liblibc/lib.rs

@@ -4999,9 +4999,36 @@ pub mod funcs {
             use types::os::arch::c95::{c_char, c_int};
             use types::os::arch::posix88::mode_t;
 
+            mod open_shim {
+                extern {
+                    #[cfg(any(target_os = "macos",
+                              target_os = "ios"))]
+                    pub fn open(path: *const ::c_char, oflag: ::c_int, ...)
+                                -> ::c_int;
+
+                    #[cfg(not(any(target_os = "macos",
+                                  target_os = "ios")))]
+                    pub fn open(path: *const ::c_char, oflag: ::c_int, mode: ::mode_t)
+                                -> ::c_int;
+                }
+            }
+
+            #[cfg(any(target_os = "macos",
+                      target_os = "ios"))]
+            #[inline]
+            pub unsafe extern fn open(path: *const c_char, oflag: c_int, mode: mode_t) -> c_int {
+                use types::os::arch::c95::c_uint;
+                open_shim::open(path, oflag, mode as c_uint)
+            }
+
+            #[cfg(not(any(target_os = "macos",
+                          target_os = "ios")))]
+            #[inline]
+            pub unsafe extern fn open(path: *const c_char, oflag: c_int, mode: mode_t) -> c_int {
+                open_shim::open(path, oflag, mode)
+            }
+
             extern {
-                pub fn open(path: *const c_char, oflag: c_int, mode: mode_t)
-                            -> c_int;
                 pub fn creat(path: *const c_char, mode: mode_t) -> c_int;
                 pub fn fcntl(fd: c_int, cmd: c_int, ...) -> c_int;
             }