Better detect Emscripten toolchain.

Emscripten is apparently changing the variables in its config file,
causing potential breakage of our build system.

Binaries of the latest/latest-upstream releases are located in a
subfolder of BINARYEN_ROOT called emscripten.
Binaries of the other releases (e.g. sdk-1.38.31-64bit) are instead
placed under the EMSCRIPTEN_ROOT folder.

This PR checks if BINARYEN_ROOT has a subfolder called emscripten, if
that does not exists, it falls back to checking the EMSCRIPTEN_ROOT.

This way we give precedence to the new releases, given that activating
multiple releases sequentially might result in having mismatching
BINARYEN_ROOT and EMSCRIPTEN_ROOT.

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