fix: Endian.ipp error on Windows for C++20 build

[iantab]

Problem

I ran into a build error using Siv3D headers under /std:c++20 on MSVC. Endian.ipp (Windows branch) uses std::byteswap, which is a C++23 library feature rather than C++20, so compilation fails with:

error C2039: 'byteswap': is not a member of 'std'

It builds fine under /std:c++latest (which is what Siv3D.vcxproj uses), so this only shows up when consuming the headers from a C++20 project.

Fix

Swap the three Windows-branch calls for the MSVC intrinsics _byteswap_ushort / _byteswap_ulong / _byteswap_uint64 from <cstdlib>. These are available regardless of /std: mode, so the Windows branch compiles cleanly under both /std:c++20 and /std:c++latest.

The macOS branch, the generic fallback, and the public Endian.hpp API are untouched.