Auto merge of #123485 - madsmtm:use-libc-copyfile, r=joboet

macOS: Use `libc` definitions for copyfile

`COPYFILE_ALL` is not yet exposed in `libc`, but the rest of what we need is, so use those definitions instead of manually defining them.

The definitions were added in rust-lang/libc#2667 and rust-lang/libc#3346.

Author: bors