This is done by introducing a new new_global_entity() function for
constructing globals/adding a visibility argument to new_alias_entity().

This flag can be set on entities where the actual address isn't
known/compared against other address. This also fixes a bug where we
used the mach-o csrings section for global char arrays which need
distinct addresses.

Default initialisation just happens to set the correct value.

mode_E is only 10 bytes wide but we should still use the 12byte wide
type_E for better alignment/consistency.