[runtime] Add a python version of the offsets tool. (mono/mono#14229)

[runtime] Add a python version of the offsets tool. (mono/mono#14229)

* [runtime] Add a python version of the offsets tool.

This version only depends on python and libclang. It currently only
works on osx.

* [runtime] Add a dummy structure needed by the python version of the offsets tool.

* [offsets-tool-py] Change some argument names for compatibility.

* [wasm] Use the python offsets tool on osx, the c# version is 32 bit, and the 64 bit version doesn't work.

* [wasm] Fix the ordering of mono libraries on the link line.

Commit migrated from https://github.com/mono/mono/commit/c2e540331d54ee6b875dd01f94e040f3888fca10