adding Ordering enum to minicore.rs, importing minicore in "tests/assembly-llvm/rust-abi-arg-attr.rs" test file
this adds the `Ordering` enum to `minicore.rs`.
consequently, this updates `tests/assembly-llvm/rust-abi-arg-attr.rs` to import `minicore` directly. previously, this test file contained traits like `Copy` `Clone` `PointeeSized`, which were giving a duplicate lang item error, so replace those by importing `minicore` completely.
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