Emscripten: Restore support for emsdk toolchain file and platform module

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