Add macOS arm64 native support

[huangminghuang]

Change Description

Adds Apple Silicon macOS support for CDT native/test builds while preserving deterministic wasm output across hosts.

Key changes:

• Add macOS arm64 native CRT support and wire native CMake logic for Mach-O arm64.
• Add an arm64 macOS GitHub Actions build using the vcpkg NuGet cache path, with a release triplet matching Linux cache behavior.
• Make host-generated wasm outputs deterministic by normalizing source paths and keeping wasm softfloat stable across macOS arm64 and Linux x86_64.
• Update native contract CMake macros so host-native contracts build as dlopen-able *_native.so modules on macOS, require C++17, preserve exported apply, and leave intrinsics unresolved for runtime lookup.
• Update CDT-owned SoftFloat consumers to use explicit softfloat32_t, softfloat64_t, and softfloat128_t names instead of the legacy float*_t aliases.
• Pin the softfloat submodule to the latest Wire-Network/berkeley-softfloat-3 master commit used by this branch.

Impact:

• Apple Silicon developers can build CDT native support locally and in CI.
• Native contract modules keep the .so suffix for platform-neutral discovery while becoming Mach-O bundles on macOS.
• Linux behavior is retained, including shared libgcc_s/libstdc++ linkage for native modules.
• CDT no longer depends on SoftFloat's legacy float*_t typedefs in native/rt code, avoiding collisions with platform headers while not forcing SOFTFLOAT_DISABLE_LEGACY_TYPE_ALIASES from CDT CMake.

API Changes

• API Changes

Adds/updates CMake-facing native contract behavior through add_native_contract(): macOS outputs now explicitly use a .so suffix and C++17 is required for host-native module compilation.

Documentation Additions

• Documentation Additions

Updates native tester compilation documentation for the macOS/native support behavior.

Testing

• Configured CDT locally with vcpkg for arm64 macOS:
  cmake -S . -B build-apple-arm64-tests -G Ninja -DCMAKE_TOOLCHAIN_FILE=vcpkg/scripts/buildsystems/vcpkg.cmake -DVCPKG_TARGET_TRIPLET=arm64-osx -DENABLE_INTEGRATION_TESTS=OFF -DENABLE_NATIVE_COMPILER=ON
• Built a CDT-only native smoke contract using add_native_contract().
• Verified the smoke output is named smoke_native.so and reports Mach-O 64-bit bundle arm64.
• Verified _apply is exported and native intrinsic symbols remain unresolved for host/runtime lookup.
• Built standalone SoftFloat with Ninja from libraries/native/softfloat.
• Reconfigured the warm arm64 macOS CDT build and built focused affected targets/objects:
  sf, rt, native/CMakeFiles/native.dir/intrinsics.cpp.obj, rt/CMakeFiles/native_rt.dir/compiler_builtins.cpp.obj
• Ran git diff --check.