kleidiai: Add SME vs SME2 distinction in kernel dispatch (#25478)

The current integration treats SME as a single capability (CPU_FEATURE_SME)
with no distinction between SME(v1) and SME2. The kernels dispatched under
CPU_FEATURE_SME use SME2-specific instructions, making dispatch incorrect
on SME(v1)-only hardware.

We introduce build-time and runtime distinction between SME and SME2, and
wire SME(v1) and SME2 kernels based on actual hardware support.

ggml/src/ggml-cpu/arch/arm/cpu-feats.cpp

@@ -28,6 +28,7 @@ struct aarch64_features {
     bool has_sve2        = false;
     bool has_i8mm        = false;
     bool has_sme         = false;
+    bool has_sme2        = false;
 
     aarch64_features() {
 #if defined(__linux__)
@@ -56,6 +57,10 @@ struct aarch64_features {
             has_sme = static_cast<bool>(oldp);
         }
 
+        if (sysctlbyname("hw.optional.arm.FEAT_SME2", &oldp, &size, NULL, 0) == 0) {
+            has_sme2 = static_cast<bool>(oldp);
+        }
+
         // Apple apparently does not implement SVE yet
 #endif
     }