Move ordinary gdbarch code to arch-utils

Move ordinary gdbarch code to arch-utils

While I think it makes sense to generate gdbarch.c, at the same time I
think it is better for ordinary code to be editable in a C file -- not
as a hunk of C code embedded in the generator.

This patch moves this sort of code out of gdbarch.sh and gdbarch.c and
into arch-utils.c, then has arch-utils.c include gdbarch.c.