Limit macOS `$INODE64` symbol names to x86 and x86_64

The new ARM-based platform doesn't need these as there's no legacy
constraints.

Tested via

**demo.c**

```c

int main() {
  fstat(0, NULL);
  fstatat(0, NULL, NULL, 0);
  lstat(NULL, NULL);
  stat(NULL, NULL);
  readdir(NULL);
  readdir_r(NULL, NULL, NULL);
}
```

**Compilation**

```none
% SDKROOT=/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.0.sdk MACOSX_DEPLOYMENT_TARGET=11.5 cc -arch arm64 demo.c

% nm a.out
0000000100008030 d __dyld_private
0000000100000000 T __mh_execute_header
                 U _fstat
                 U _fstatat
                 U _lstat
0000000100003e64 T _main
                 U _readdir
                 U _readdir_r
                 U _stat
                 U dyld_stub_binder
```

This has also been experimentally compiled on a Developer Transition Kit.

src/unix/mod.rs

@@ -685,7 +685,10 @@ extern "C" {
     )]
     pub fn fchmod(fd: ::c_int, mode: mode_t) -> ::c_int;
 
-    #[cfg_attr(target_os = "macos", link_name = "fstat$INODE64")]
+    #[cfg_attr(
+        all(target_os = "macos", not(target_arch = "aarch64")),
+        link_name = "fstat$INODE64"
+    )]
     #[cfg_attr(target_os = "netbsd", link_name = "__fstat50")]
     #[cfg_attr(
         all(target_os = "freebsd", any(freebsd11, freebsd10)),
@@ -695,7 +698,10 @@ extern "C" {
 
     pub fn mkdir(path: *const c_char, mode: mode_t) -> ::c_int;
 
-    #[cfg_attr(target_os = "macos", link_name = "stat$INODE64")]
+    #[cfg_attr(
+        all(target_os = "macos", not(target_arch = "aarch64")),
+        link_name = "stat$INODE64"
+    )]
     #[cfg_attr(target_os = "netbsd", link_name = "__stat50")]
     #[cfg_attr(
         all(target_os = "freebsd", any(freebsd11, freebsd10)),
@@ -738,7 +744,10 @@ extern "C" {
     #[cfg_attr(target_os = "netbsd", link_name = "__opendir30")]
     pub fn opendir(dirname: *const c_char) -> *mut ::DIR;
 
-    #[cfg_attr(target_os = "macos", link_name = "readdir$INODE64")]
+    #[cfg_attr(
+        all(target_os = "macos", not(target_arch = "aarch64")),
+        link_name = "readdir$INODE64"
+    )]
     #[cfg_attr(target_os = "netbsd", link_name = "__readdir30")]
     #[cfg_attr(
         all(target_os = "freebsd", any(freebsd11, freebsd10)),
@@ -774,7 +783,10 @@ extern "C" {
         group: ::gid_t,
         flags: ::c_int,
     ) -> ::c_int;
-    #[cfg_attr(target_os = "macos", link_name = "fstatat$INODE64")]
+    #[cfg_attr(
+        all(target_os = "macos", not(target_arch = "aarch64")),
+        link_name = "fstatat$INODE64"
+    )]
     #[cfg_attr(
         all(target_os = "freebsd", any(freebsd11, freebsd10)),
         link_name = "fstatat@FBSD_1.1"
@@ -1007,7 +1019,10 @@ extern "C" {
         ifname: *mut ::c_char,
     ) -> *mut ::c_char;
 
-    #[cfg_attr(target_os = "macos", link_name = "lstat$INODE64")]
+    #[cfg_attr(
+        all(target_os = "macos", not(target_arch = "aarch64")),
+        link_name = "lstat$INODE64"
+    )]
     #[cfg_attr(target_os = "netbsd", link_name = "__lstat50")]
     #[cfg_attr(
         all(target_os = "freebsd", any(freebsd11, freebsd10)),
@@ -1483,7 +1498,8 @@ cfg_if! {
                        link_name = "fdopendir$INODE64$UNIX2003")]
             pub fn fdopendir(fd: ::c_int) -> *mut ::DIR;
 
-            #[cfg_attr(target_os = "macos", link_name = "readdir_r$INODE64")]
+            #[cfg_attr(all(target_os = "macos", not(target_arch = "aarch64")),
+                       link_name = "readdir_r$INODE64")]
             #[cfg_attr(target_os = "netbsd", link_name = "__readdir_r30")]
             #[cfg_attr(
                 all(target_os = "freebsd", any(freebsd11, freebsd10)),