Use objcopy from rust's llvm-tools-preview?

[mx-shift]

Rust ships llvm-tools-preview (rust-lang/rust#85658) which includes llvm-objcopy. This component should be installable via rust-toolchain.yaml. Using this in xtask would remove the need for installing host system packages.

[steveklabnik]

iirc @cbiffle had said that the llvm version of these tools doesn't do the right things for some stuff we need. I don't remember more context than that.

[steveklabnik]

I did find ac53573

[cbiffle]

I believe we've recently ended our reliance on objcopy in general so we can close this.