Enable outline-atomics by default on AArch64 Android

Per LLVM commit c5e7e649d537 ("[AArch64][Clang][Linux] Enable
out-of-line atomics by default.") [1], Clang enables these on Android.
Thus, do the same in Rust.

[1]: https://github.com/llvm/llvm-project/commit/c5e7e649d537067de

compiler/rustc_target/src/spec/targets/aarch64_linux_android.rs

@@ -21,7 +21,7 @@ pub(crate) fn target() -> Target {
             max_atomic_width: Some(128),
             // As documented in https://developer.android.com/ndk/guides/cpu-features.html
             // the neon (ASIMD) and FP must exist on all android aarch64 targets.
-            features: "+v8a,+neon".into(),
+            features: "+v8a,+neon,+outline-atomics".into(),
             // the AAPCS64 expects use of non-leaf frame pointers per
             // https://github.com/ARM-software/abi-aa/blob/4492d1570eb70c8fd146623e0db65b2d241f12e7/aapcs64/aapcs64.rst#the-frame-pointer
             // and we tend to encounter interesting bugs in AArch64 unwinding code if we do not