Extend commit 644ad91e6a (Emscripten: Restore support for emsdk
toolchain file and platform module, 2025-09-17) to support using a
third-party `CMAKE_TOOLCHAIN_FILE` that includes the emsdk-provided
toolchain file.
Issue: #27232
Emscripten provides a combined toolchain file and platform module that
predates CMake upstream support. Since commit 96d9b94a98 (Emscripten:
Add platform modules, 2025-05-16), our `Platform/Emscripten` module is
included instead, suppressing existing behavior for users that set
`CMAKE_TOOLCHAIN_FILE` to the emsdk-provided toolchain file. Restore
the previous behavior by teaching our module to defer to the original
module in that case.
Issue: #27232
Brad King