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