Enable outline-atomics by default on AArch64 FreeBSD

Clang does not currently have this enabled on FreeBSD, but there doesn't
seem to be any specific reason not to. Thus, enable it here.

Author: tgross35

compiler/rustc_target/src/spec/targets/aarch64_unknown_freebsd.rs

@@ -15,7 +15,7 @@ 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,
             supported_sanitizers: SanitizerSet::ADDRESS