Pass +atomics-32 feature for thumbv6m target

https://reviews.llvm.org/D120026 changed atomics on thumbv6m to
use libatomic, to ensure that atomic load/store are compatible with
atomic RMW/CAS. However, Rust wants to expose only load/store
without libcalls.

https://reviews.llvm.org/D130480 added support for this behind
the +atomics-32 target feature, so enable that feature.

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -440,6 +440,8 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
             .features
             .split(',')
             .filter(|v| !v.is_empty() && backend_feature_name(v).is_some())
+            // Drop +atomics-32 feature introduced in LLVM 15.
+            .filter(|v| *v != "+atomics-32" || get_version() >= (15, 0, 0))
             .map(String::from),
     );