Finalize Aarch64 support

Update platform support for linux to avoid the use of syscalls not supported by Aarch64.

• link, chown, and open should use fchownat, linkat, and openat with fd set to AT_FDCWD.
• use dup3 instead of dup2.

NB: This changes the behaviour for all architectures

After updating va_list to my branch (thepowersgang/va_list-rs#9) that supports aarch64 I was able to run the tests.