Enable outline-atomics for aarch64-unknown-linux-musl
They were disabled in bd287fa508 and haven't been causing problems for a while anymore, see https://github.com/rust-lang/rust/issues/89626 for details.
The entire testsuite still passes on `aarch64-unknown-linux-musl` with this feature enabled.
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.