Split gdbarch.h into two files

This patch splits gdbarch.h into two files -- gdbarch.h now is
editable and hand-maintained, and the new gdbarch-gen.h file is the
only thing generated by gdbarch.sh.  This lets us avoid maintaining
boilerplate in the gdbarch.sh file.

Note that gdbarch.sh still generates gdbarch.h after this patch.  This
makes it easier to re-run when rebasing.  This code is removed in a
subsequent patch.