Enable outline-atomics by default on AArch64 Fuchsia

Clang has done this by default since LLVM commit 1a963d3 ("[Driver]
Make -moutline-atomics default for aarch64-fuchsia targets"), [1], so do
the same here.

[1]: llvm/llvm-project@1a963d3

Author: tgross35

compiler/rustc_target/src/spec/targets/aarch64_unknown_fuchsia.rs

@@ -5,7 +5,7 @@ use crate::spec::{
 pub(crate) fn target() -> Target {
     let mut base = base::fuchsia::opts();
     base.cpu = "generic".into();
-    base.features = "+v8a,+crc,+aes,+sha2,+neon".into();
+    base.features = "+v8a,+crc,+aes,+sha2,+neon,+outline-atomics".into();
     base.max_atomic_width = Some(128);
     base.stack_probes = StackProbeType::Inline;
     base.supported_sanitizers = SanitizerSet::ADDRESS