[a64] Implement FEAT_LSE optimizations

Adds `platform_arm64` similar to `platform_arm64` to populate a 64-bit feature-flag value and creates a `a64_extension_mask` configuration-mask for the detection and utilization of certain arm ISA features. The first feature being `FEAT_LSE` which maps to `XBYAK_AARCH64_HWCAP_ATOMIC`.

Just like the x64 backend, implements `IsFeatureEnabled` to the emitter for checking if the host processor supports a certain subset of feature flag(s) at once.

Optimizes `OPCODE_ATOMIC_EXCHANGE`, `OPCODE_ATOMIC_COMPARE_EXCHANGE`,
and `OPCODE_RESERVED_STORE` with `FEAT_LSE` instructions.

src/xenia/base/platform.h

@@ -186,5 +186,7 @@ constexpr char kGuestPathSeparator = '\\';
 }  // namespace xe
 #if XE_ARCH_AMD64 == 1
 #include "platform_amd64.h"
+#elif XE_ARCH_ARM64 == 1
+#include "platform_arm64.h"
 #endif
 #endif  // XENIA_BASE_PLATFORM_H_

src/xenia/base/platform_arm64.cc

@@ -0,0 +1,50 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2026 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#include "xenia/base/cvar.h"
+#include "xenia/base/platform.h"
+#define XBYAK_NO_OP_NAMES
+#include "third_party/xbyak_aarch64/xbyak_aarch64/xbyak_aarch64.h"
+#include "third_party/xbyak_aarch64/xbyak_aarch64/xbyak_aarch64_util.h"
+DEFINE_int64(a64_extension_mask, -1LL,
+             "Allow the detection and utilization of specific instruction set "
+             "features.\n"
+             "    0 = armv8.0\n"
+             "    1 = Large System Extensions(LSE) atomic operations\n"
+             "   -1 = Detect and utilize all possible processor features\n",
+             "a64");
+namespace xe {
+namespace arm64 {
+static uint64_t g_feature_flags = 0U;
+static bool g_did_initialize_feature_flags = false;
+uint64_t GetFeatureFlags() {
+  if (!g_did_initialize_feature_flags) {
+    InitFeatureFlags();
+  }
+  return g_feature_flags;
+}
+XE_COLD
+XE_NOINLINE
+void InitFeatureFlags() {
+  uint64_t feature_flags_ = 0U;
+  {
+    Xbyak_aarch64::util::Cpu cpu_;
+#define TEST_EMIT_FEATURE(emit, ext)                \
+  if ((cvars::a64_extension_mask & emit) == emit) { \
+    feature_flags_ |= (cpu_.has(ext) ? emit : 0);   \
+  }
+    TEST_EMIT_FEATURE(kA64EmitLSE,
+                      Xbyak_aarch64::util::XBYAK_AARCH64_HWCAP_ATOMIC);
+#undef TEST_EMIT_FEATURE
+  }
+  g_feature_flags = feature_flags_;
+  g_did_initialize_feature_flags = true;
+}
+}  // namespace arm64
+}  // namespace xe

src/xenia/base/platform_arm64.h

@@ -0,0 +1,29 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2026 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_BASE_PLATFORM_ARM64_H_
+#define XENIA_BASE_PLATFORM_ARM64_H_
+#include <cstdint>
+
+namespace xe {
+namespace arm64 {
+enum A64FeatureFlags : uint64_t {
+  kA64EmitLSE = 1 << 0,
+
+};
+
+XE_NOALIAS
+uint64_t GetFeatureFlags();
+XE_COLD
+void InitFeatureFlags();
+
+}  // namespace arm64
+}  // namespace xe
+
+#endif  // XENIA_BASE_PLATFORM_ARM64_H_