We are locked to version 1 of the ABI, so don't load what might be a symlink to a newer and incompatible version.
Platforms like OpenBSD that only have .so versions of libraries can add a special case here like Android in the future.
Fixes https://github.com/libsdl-org/SDL/issues/15172
When the OpenXR loader library fails to load, provide a detailed error
message explaining:
- On Windows: need openxr_loader.dll in app directory or PATH
- On Linux: install libopenxr-loader package or set LD_LIBRARY_PATH
- Can use SDL_HINT_OPENXR_LIBRARY hint to specify path
Also update D3D12 backend to include the error in its warning log,
and remove redundant SDL_SetError in Vulkan backend that was
overwriting the detailed message.
