[mini] Add skeleton WebAssembly mini backend. It's not functional and some code needs...

[mini] Add skeleton WebAssembly mini backend. It's not functional and some code needs to be shuffled to better places.

The wasm backend is inspired on the x86 one with a few key differences:

1) It assumes a different aot runtime will be used to deal with all the weirdness of wasm.
In particular, it can run under MONO_AOT_MODE_INTERP_LLVMONLY without an actual working AOT compiler.

2) It's the only non-ISA based backend, so lots of weird things.
Like, no registers or addressable stack - for realsies.