[msvc] Add WinRT ARM64 support and fix toolchain pathing/compilation issues

[FixFlare]

Summary

This PR adds support for building WinRT (UWP) applications for the ARM64 architecture using MSVC. It provides a new environment setup script and fixes directory-naming bugs in hxcpp.

Key Changes

1. New Setup Script: msvc-winrt-arm64-setup.bat

Added a dedicated setup script for WinRT ARM64. It specifically:

• Uses the x64_arm64 cross-compiler (host x64, target arm64).
• Configures the UWP app platform via vcvarsall.bat.
• Automates detection of the ARM64 compiler components via vswhere.

2. BINDIR Routing for WinRT ARM64

Updated BuildTool.hx to set BINDIR to WinRTArm64 when the winrt and HXCPP_ARM64 defines are both present. This ensures compatibility with engine-level expectations (like Lime) and prevents WinRT ARM64 binaries from being placed in the x64 (WinRT64) folder.

3. Object Directory Naming

Fixed a bug in msvc-toolchain.xml where ARM64 builds would receive a redundant arm prefix (e.g., armarm64). It also ensures the -rt (WinRT) suffix is preserved and not overwritten by the architecture flag.

4. Core Runtime Hardening (Fixes ARM64 Compilation Errors)

• hxString.h: Added explicit to WinRT-specific string constructors (Platform::String^). This fixes "ambiguous overload" errors that occur on the ARM64 MSVC compiler during implicit conversion.
• StdLibs.cpp: Optimized __trace to use HX_CSTRING for static string literals. This fixes compilation failures related to string operator ambiguity on ARM64 and prevents potential runtime crashes in the UWP sandbox.

Verification

• Verified successful cross-compilation of Haxe applications for ARM64 UWP on an x64 host.
• Verified correct directory structure generation for native libraries and object files.
• Verified manifest validation success with the resulting binaries.