gdb: Update gdbarch.py with latest changes in gdbarch.c

Commit 2b16913c ("gdb: make gdbarch_alloc take ownership of the tdep")
changed gdbarch.c without updating gdbarch.py.  As a result, running
gdbarch.py reverts those changes and causes the build to fail.

So change gdbarch.py to generate the current version of gdbarch.c.

Approved-By: Simon Marchi <simon.marchi@efficios.com>