ABI checks: add support for loongarch

LoongArch psABI[^1] specifies that LSX vector types are passed via general-purpose
registers, while LASX vector types are passed indirectly through the stack.

This patch addresses the following warnings:

```
warning: this function call uses a SIMD vector type that is not currently supported with the chosen ABI
    --> .../library/core/src/../../stdarch/crates/core_arch/src/loongarch64/lsx/generated.rs:3695:5
     |
3695 |     __lsx_vreplgr2vr_b(a)
     |     ^^^^^^^^^^^^^^^^^^^^^ function called here
     |
     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     = note: for more information, see issue #116558 <#116558>
     = note: `#[warn(abi_unsupported_vector_types)]` on by default
```

[^1]: https://github.com/loongson/la-abi-specs/blob/release/lapcs.adoc

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

@@ -36,7 +36,10 @@ fn do_check_abi<'tcx>(
     target_feature_def: DefId,
     mut emit_err: impl FnMut(Option<&'static str>),
 ) {
-    let feature_def = tcx.sess.target.features_for_correct_vector_abi();
+    let Some(feature_def) = tcx.sess.target.features_for_correct_vector_abi() else {
+        // Target doesn't use vector registers for vectors.
+        return;
+    };
     let codegen_attrs = tcx.codegen_fn_attrs(target_feature_def);
     for arg_abi in abi.args.iter().chain(std::iter::once(&abi.ret)) {
         let size = arg_abi.layout.size;

compiler/rustc_target/src/target_features.rs

@@ -625,24 +625,27 @@ impl super::spec::Target {
         }
     }
 
-    pub fn features_for_correct_vector_abi(&self) -> &'static [(u64, &'static str)] {
+    // Returns None if the given target does not use vector registers to pass vector-type data.
+    pub fn features_for_correct_vector_abi(&self) -> Option<&'static [(u64, &'static str)]> {
         match &*self.arch {
-            "x86" | "x86_64" => X86_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "aarch64" | "arm64ec" => AARCH64_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "arm" => ARM_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "powerpc" | "powerpc64" => POWERPC_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "loongarch64" => &[], // on-stack ABI, so we complain about all by-val vectors
-            "riscv32" | "riscv64" => RISCV_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "wasm32" | "wasm64" => WASM_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "s390x" => S390X_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "sparc" | "sparc64" => SPARC_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "hexagon" => HEXAGON_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "mips" | "mips32r6" | "mips64" | "mips64r6" => MIPS_FEATURES_FOR_CORRECT_VECTOR_ABI,
-            "bpf" => &[], // no vector ABI
-            "csky" => CSKY_FEATURES_FOR_CORRECT_VECTOR_ABI,
+            "x86" | "x86_64" => Some(X86_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "aarch64" | "arm64ec" => Some(AARCH64_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "arm" => Some(ARM_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "powerpc" | "powerpc64" => Some(POWERPC_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "loongarch64" => None, // LoongArch doesn't use vector registers for vectors
+            "riscv32" | "riscv64" => Some(RISCV_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "wasm32" | "wasm64" => Some(WASM_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "s390x" => Some(S390X_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "sparc" | "sparc64" => Some(SPARC_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "hexagon" => Some(HEXAGON_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "mips" | "mips32r6" | "mips64" | "mips64r6" => {
+                Some(MIPS_FEATURES_FOR_CORRECT_VECTOR_ABI)
+            }
+            "bpf" => Some(&[]), // no vector ABI
+            "csky" => Some(CSKY_FEATURES_FOR_CORRECT_VECTOR_ABI),
             // FIXME: for some tier3 targets, we are overly cautious and always give warnings
             // when passing args in vector registers.
-            _ => &[],
+            _ => Some(&[]),
         }
     }