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.

Author: nikic

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),
     );
 

compiler/rustc_target/src/spec/thumbv6m_none_eabi.rs

@@ -13,7 +13,9 @@ pub fn target() -> Target {
             abi: "eabi".into(),
             // The ARMv6-M architecture doesn't support unaligned loads/stores so we disable them
             // with +strict-align.
-            features: "+strict-align".into(),
+            // Also force-enable 32-bit atomics, which allows the use of atomic load/store only.
+            // The resulting atomics are ABI incompatible with atomics backed by libatomic.
+            features: "+strict-align,+atomics-32".into(),
             // There are no atomic CAS instructions available in the instruction set of the ARMv6-M
             // architecture
             atomic_cas: false,