Add new gdbarch generator

Add new gdbarch generator

The new gdbarch generator is a Python program.  It reads the
"components.py" that was created in the previous patch, and generates
gdbarch.c and gdbarch-gen.h.

This is a relatively straightforward translation of the existing .sh
code.  It doesn't try very hard to be idiomatic Python or to be
especially smart.

It is, however, incredibly faster:

    $ time ./gdbarch.sh

    real	0m8.197s
    user	0m5.779s
    sys	0m3.384s

    $ time ./gdbarch.py

    real	0m0.065s
    user	0m0.053s
    sys	0m0.011s

Co-Authored-By: Tom Tromey <tom@tromey.com>