Limit macOS $INODE64 symbol names to x86 and x86_64

[shepmaster]

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

Tested via

demo.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

% 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.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[JohnTitor]

r=me once the style issue that CI reported is fixed.

[shepmaster]

style issue

Addressed!

[JohnTitor]

Thanks!