[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/app/xenia_main.cc

@@ -492,6 +492,8 @@ bool EmulatorApp::OnInitialize() {
 
 #if XE_ARCH_AMD64 == 1
   amd64::InitFeatureFlags();
+#elif XE_ARCH_ARM64 == 1
+  arm64::InitFeatureFlags();
 #endif
 
   std::filesystem::path content_root = cvars::content_root;