Enable outline-atomics for aarch64-unknown-linux-musl

They were disabled in bd287fa508 and haven't been
causing problems for a while anymore.

The entire testsuite still passes on aarch64-unknown-linux-musl with this feature
enabled.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

compiler/rustc_target/src/spec/targets/aarch64_unknown_linux_musl.rs

@@ -6,7 +6,7 @@ pub(crate) fn target() -> Target {
     let mut base = base::linux_musl::opts();
     base.max_atomic_width = Some(128);
     base.supports_xray = true;
-    base.features = "+v8a".into();
+    base.features = "+v8a,+outline-atomics".into();
     base.stack_probes = StackProbeType::Inline;
     base.supported_sanitizers = SanitizerSet::ADDRESS
         | SanitizerSet::CFI