aarch64: Make outline-atomics a known target feature

This is a feature used by LLVM that is enabled for our `aarch64-linux`
targets, which we would like to configure on in `std`. Thus, mark
`outline-atomics` a known feature. It is left unstable for now.

Author: tgross35

compiler/rustc_target/src/target_features.rs

@@ -248,6 +248,9 @@ static AARCH64_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("mte", Stable, &[]),
     // FEAT_AdvSimd & FEAT_FP
     ("neon", Stable, &[]),
+    // Backend option that we would like to `cfg` on in library, unstable since it
+    // doesn't need to user-togglable.
+    ("outline-atomics", Unstable(sym::aarch64_unstable_target_feature), &[]),
     // FEAT_PAUTH (address authentication)
     ("paca", Stable, &[]),
     // FEAT_PAUTH (generic authentication)

tests/assembly-llvm/asm/aarch64-outline-atomics.rs

@@ -8,6 +8,10 @@
 use std::sync::atomic::AtomicI32;
 use std::sync::atomic::Ordering::*;
 
+// Verify config on outline-atomics works (it is always enabled on aarch64-linux).
+#[cfg(not(target_feature = "outline-atomics"))]
+compile_error!("outline-atomics is not enabled");
+
 pub fn compare_exchange(a: &AtomicI32) {
     // On AArch64 LLVM should outline atomic operations.
     // CHECK: __aarch64_cas4_relax