Enable outline-atomics by default on AArch64 OpenBSD

Clang has recently started doing this, as of LLVM commit 5d774ec8d183
("[Driver] Enable outline atomics for OpenBSD/aarch64") [1]. Thus, do
the same here.

[1]: 5d774ec8d1

compiler/rustc_target/src/spec/targets/aarch64_unknown_openbsd.rs

@@ -13,7 +13,7 @@ pub(crate) fn target() -> Target {
         data_layout: "e-m:e-p270:32:32-p271:32:32-p272:64:64-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128-Fn32".into(),
         arch: Arch::AArch64,
         options: TargetOptions {
-            features: "+v8a".into(),
+            features: "+v8a,+outline-atomics".into(),
             max_atomic_width: Some(128),
             stack_probes: StackProbeType::Inline,
             ..base::openbsd::opts()