We cannot reuse the Sub in case of overflows.

This fixes backend/sparc_cmp2.c.
merge gen_ir.py and gen_ir_io.py scripts

There was a lot of duplicated code, some changes in the templates allows
to share the two generators.
cleanup, use C99
cleanup, use C99
improve docu for be_parse_arg
Correct default_layout_compound_type().

The values used for offset were just plain wrong.
cleanup spec by introducing new Attribute object
generate is_XXX and getters/setters for abstract node bases

= is_entconst(), is_typeconst, get_entconst_entity() and
get_typeconst_type()