asm2wasm: handle overloaded emscripten imports

[kripken]

We already fixed emscripten_asm_const to be type-signatured, but looks like the same happens for cxa_find_matching_catch. Not sure if the best fix is in emscripten or here.

cc @AndrewScheidecker

[kripken]

In particular, we need this for C++ exceptions support.

[kripken]

Looks like this is very specific to that cxa_find_matching_catch method, so I fixed this in emscripten. It will now emit cxa_find_matching_catch_X for X the number of arguments, thus avoiding overloading that method.