Build under MSYS2 failed

⚠️ **Issues not using this template will be systematically closed.**

**Describe the bug**

Filament cannot be built under MSYS2 (MinGW-w64) on Windows. While CMake configuration succeeds, the build fails at compile time due to:

1. The bluegl library requires the MASM assembler (`ml`/`ml64.exe`) which is not available under MSYS2.
2. Several source files use `#if defined(WIN32)` guards which are not triggered because MinGW Clang does not define the `WIN32` macro (only `_WIN32`). This causes the wrong code paths to be taken throughout the codebase.

**To Reproduce**

1. Install MSYS2 with the UCRT64 environment:
   ```sh
   pacman -S mingw-w64-ucrt-x86_64-toolchain mingw-w64-ucrt-x86_64-clang cmake ninja
   ```
2. Clone Filament and configure:
   ```sh
   cmake -B build -G Ninja \
     -DCMAKE_C_COMPILER=clang \
     -DCMAKE_CXX_COMPILER=clang++
   ```
   Configuration succeeds.

3. Build:
   ```sh
   cmake --build build
   ```
   Build fails with multiple errors.

**Expected behavior**

Filament should either:
- Support building with MinGW-w64 (MSYS2) toolchains, or
- Explicitly reject MinGW at the compiler check stage with a clear error message.

**Screenshots**

N/A

**Logs**

--- bluegl: MASM assembler missing ---
```
[1/4] Building ASM_MASM object libs/bluegl/CMakeFiles/bluegl.dir/src/BlueGLCoreWindowsImpl.S.obj
FAILED: libs/bluegl/CMakeFiles/bluegl.dir/src/BlueGLCoreWindowsImpl.S.obj
ml ... -c -Fo ... D:/filament/libs/bluegl/src/BlueGLCoreWindowsImpl.S
CreateProcess failed: The system cannot find the file specified.
```

--- CallStack.cpp: execinfo.h not found (WIN32 not defined by MinGW Clang) ---
```
libs/utils/src/CallStack.cpp:31:10: fatal error: 'execinfo.h' file not found
   31 | #include <execinfo.h>
```

--- memalign.h: posix_memalign not found (WIN32 not defined by MinGW Clang) ---
```
libs/utils/include/utils/memalign.h:43:14: error: no member named 'posix_memalign' in the global namespace
   43 |     (void) ::posix_memalign(&p, align, size);
```

--- ashmem.cpp: MSVC-specific CRT functions not available ---
```
libs/utils/src/ashmem.cpp:155:35: error: cannot initialize a parameter of type 'wchar_t *'
    155 |     const char* tmpPath = _mktemp(template_path);
libs/utils/src/ashmem.cpp:156:29: error: use of undeclared identifier '_O_BINARY'
    156 |     int fd = _open(tmpPath, _O_BINARY);
libs/utils/src/ashmem.cpp:159:9: error: use of undeclared identifier '_chsize'
    159 |     if (_chsize(fd, size) == -1) {
```

**Desktop (please complete the following information):**

 - OS: Windows 11 64-bit (MSYS2 UCRT64)
 - GPU: NVIDIA GeForce RTX 4060 Max-Q (8GB VRAM)
 - Backend: Vulkan

**Additional context**

**Root cause: `WIN32` vs `_WIN32`**

Under MSVC, the compiler defines `WIN32`. Under MinGW Clang, only `_WIN32`, `__WIN32`, and `__WIN32__` are defined — `WIN32` (without underscore) is NOT defined:

```
$ echo 'int x;' | clang++ -std=c++20 -dM -E -x c++ - | grep -i win32
#define _WIN32 1
#define __WIN32 1
#define __WIN32__ 1
```

Filament uses `#if defined(WIN32)` in multiple places. Under MinGW these guards silently fail, causing the code to take Unix/POSIX code paths:

| File | Guard | Effect |
|------|-------|--------|
| `libs/utils/include/utils/memalign.h:40` | `#if defined(WIN32)` | Falls through to `posix_memalign()` which MinGW does not provide |
| `libs/utils/src/CallStack.cpp:30` | `#if !defined(WIN32)` | Includes `<execinfo.h>` (glibc-only) instead of using the Windows stub |
| `libs/utils/src/ashmem.cpp:150` | (implicit via WIN32) | Takes the MSVC CRT branch (`_mktemp`, `_chsize`) instead of the POSIX branch (`mkstemp`) |
| `filament/backend/src/opengl/platforms/PlatformWGL.cpp:21` | `#ifdef _MSC_VER` | `FILAMENT_PLATFORM_WGL` is not defined for MinGW |

**bluegl MASM requirement**

`libs/bluegl/CMakeLists.txt:13-16` enables MASM on 64-bit Windows:
```cmake
if (WIN32 AND IS_64_BIT)
    enable_language(ASM_MASM)
    set_property(SOURCE src/BlueGLCoreWindowsImpl.S PROPERTY LANGUAGE ASM_MASM)
endif()
```
CMake finds a virtual `ml` entry but the binary does not exist in MSYS2, so it fails at build time with `CreateProcess failed`.

**Why build on MSYS2?**
I'm porting a [cross-platform project](https://codeberg.org/taisho-guy/AviQtl) to Windows, and the author made a MSYS2 build script. While it's possible to use MSVC to build the whole project, the build process just got failed on Filament, and other libraries have no issues.

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