implement better availability probing for copy_file_range
Followup to https://github.com/rust-lang/rust/pull/75428#discussion_r469616547
Previously syscall detection was overly pessimistic. Any attempt to copy to an immutable file (EPERM) would disable copy_file_range support for the whole process.
The change tries to copy_file_range on invalid file descriptors which will never run into the immutable file case and thus we can clearly distinguish syscall availability.
e413d89aa7