[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_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_