Based on the discussions in the AOT embedded improvements RFC, this adds a flag to the target which changes the internal operators to an unpacked API. The unpacked API spreads the input buffers across the operator function, for example:

int32_t operator(void* arg0, void* arg1);

As opposed to the traditional packed API:

int32_t operator(void** args);

Uneffected is the entrypoint function, which retains a packed API for
compatibility with other parts of TVM. This is done by changing the
passes taken by none entrypoint (CallingConv::kEntryPoint) functions.

This removes the logic for deciding the entrypoint from the compiler
passes and instead moves it into the metadata code generation. By moving
the generation, we can generate a variety of entrypoints on top of the
compiler output (such as the micro entrypoint discussed in the RFC).

…ions

(Also contains minor clean up of output variables)

(Also moves the entrypoint name to a constant)