[Build] Linux→Windows cross-compile toolchain (clang-cl + xwin)

New CMake preset `cross-win-clangcl` for Ninja Multi-Config on
non-Windows hosts. Toolchain: clang-cl (MSVC-ABI), lld-link, xwin SDK/CRT
splat. Shader compilation routes through wine fxc.exe with FXC_PATH
env forwarding and unix→Windows path translation via `winepath -w`.
Plus per-file build fixes (constexpr→const, llvm-rc forward-slash,
zlib-ng AVX guard, /RTCsu MSVC-only). third_party/snappy bumped to
fabi/sylpheed-crossbuild for the target-aware POSIX-gate header.

Produces `xenia_canary.exe` (Debug MSVC) suitable for use as the
audit oracle under Wine for xenia-rs work. See
docs/CROSS_BUILD_SETUP.md for the full reproduction recipe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

third_party/CMakeLists.txt

@@ -332,7 +332,7 @@ else()
     X86_AVX512VNNI
     WITH_GZFILEOP
   )
-  if(NOT MSVC)
+  if(NOT MSVC OR CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
     # GCC/Clang have native __builtin_ctz/__builtin_ctzll (MSVC gets these
     # from fallback_builtins.h). Defining these enables optimized compare256
     # and longest_match codepaths.