Generate new gdbarch-components.py from gdbarch.sh

The new gdbarch.sh approach will be to edit a Python file, rather than
adding a line to a certain part of gdbarch.sh.  We use the existing sh
code, though, to generate the first draft of this .py file.

Documentation on the format will come in a subsequent patch.

Note that some info (like "staticdefault") in the current code is
actually unused, and so is ignored by this new generator.