Emscripten: Towards a working C++ UNO bridge HEADmaster

...by making the general UNO -> C++ function call case work (modulo handling
exceptions, which I'll look into later).

Wasm call_indirect unfortunately needs to statically know the call target's
signature, so we statically need to know all possible signatures.  So introduce
wasmcallgen helper to scan the existing UNOIDL API upfront and generate the
relevant callvirtualfunction-wrapper.cxx and callvirtualfunction-inst.s code.
(The good thing is that the number of different signatures is somewhat limited,
each parameter can only be one of i32, i64, f32, or f64.  So even if 3rd-party
extensions bring along new UNOIDL interface methods, chances are relatively high
that the signatures needed for them would already be covered by the existing
ones.)

Testing this can nicely be done in unotest/source/embindtest/embindtest.js via
css.script.Invocation (which internally exercises the relevant code).  (Instead
of adding individual org.libreoffice.embindtest.StructLong/String etc., it would
be nicer to introduce some polymorphic StructOne<T>, but the Emind UNO binding
does not support polymorphic structs, so the embindtest.js code would not work.)

Change-Id: If829c9e3772bfd27561f3ad743d38a71d11545b6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/167308
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>
Tested-by: Jenkins

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