Tooling/Add experimental clang-cl cross-build

[Belonit]

Summary

Add an experimental Linux-hosted Win32 cross-build using clang-cl with the MSVC
ABI, headers, and libraries. The branch also makes the C++ sources and legacy
assembly accepted by clang-cl, aligns bgfx render-record allocations, and adds a
VS Code IntelliSense example.

Behavior and compatibility

• Supported Visual Studio 2022 Win32 Debug and Release behavior is preserved.
• The clang-cl configuration is unsupported and does not expand the build matrix.
• Therenderer translation unit now defines BX_CONFIG_DEBUG to match the bgfx libraries and allocates bgfx records alignment-correctly, keeping MSVC and clang-cl builds consistent.

The codebase may still contain undefined behavior the supported MSVC build does not expose, so a successful clang-cl build is not runtime evidence.
Validate any clang-cl result separately.

Known clang-cl runtime observations:

• Without fix: UB in movement path advancement #76, existing undefined behavior causes the game to crash.
• Without fix: voxel cache rendering corruption #77, existing undefined behavior produces rendering artifacts on voxels.

Validation

• clang-cl 22.1.8, MSVC 14.44.35207, Windows SDK 10.0.26100.0: configured and
  built (Release, Ninja) with inherited warnings and no errors; LogStress passed, including 100,000 intact log lines.
• Supported MSVC Win32 Release: build completed under Wine with cl 14.44.35207 (Ninja), including the test executables. The Visual Studio Debug configuration and CI were not exercised.

Documentation

docs/BUILDING.md documents the experimental toolchain requirements,
configuration, and the runtime limitation.

Checklist

• The change is focused; unrelated mechanical cleanup is separate
• Compatibility effects and any migration are explicit
• Documentation impact is stated
• Validation distinguishes what passed, failed, and was not run
• No prohibited assets, binaries, SDKs, credentials, or generated output are included

[github-actions]

Development builds of e1391a5:

• opents-Release-8fe492e
• opents-Debug-8fe492e

The links work without a GitHub account. Artifacts expire after 90 days, and this comment follows the latest successful build.

[tomsons26]

Old cdecl, register keyword and asm issues could be handled in another PR so that get's merged in faster.
Never meant to leave cdecl and register but forgot about them.

[ZivDero]

Okay should be smaller now

[ZivDero]

BX_CONFIG_DEBUG + allocator could also be their own PR. Arguably even a fix in general.

[Belonit]

Updated.
General source and bgfx fixes are now merged separately, so this PR only contains the clang-cl tooling and VS Code configuration.