[Hexagon] Delete offload runtime, move files to right places

[kparzysz-quic]

Within src/runtime/hexagon

• delete directory android,
• move files from hexagon to ., delete hexagon,
• merge host/hexagon_module.cc with hexagon_module.cc, delete host.

Rename HexagonHostModuleNode to HexagonModuleNode.

Edit:
One additional change in the contents of HexagonModule is that the list of functions with the "C packed" ABI is no longer needed and has been removed. Functions that were offloaded to Hexagon could have had any number of parameters, and the interface would be created inside of TVM when the offloaded code was extracted into its own function. Calls to these functions could not be generated by TVM at the same time, because the calls were placed in the host code, and were "remote calls", i.e. were calling the runtime to indicate what functions need to be called on Hexagon. The actual "call" instructions were in the runtime, which needed to know what parameter-passing conventions was used: it would set up parameter register, stack, and execute a branch to the function code. Because of that, the list of functions following the "C packed" convention was also a member of the Hexagon module.
With the offload removed, there are no "remote calls" anymore, and the calling conventions are handled internally by LLVM.

cc @mehrdadh

[mehrdadh]

@kparzysz-quic this is great!
I think you missed a header file path change.

[mehrdadh]

@kparzysz-quic can you add notes in the PR description about changes in hexagon codegen for better documentation?

[mehrdadh]

@kparzysz-quic there's a merge conflict which might have been introduced by #11065. Please rebase and then we can merge.