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This is not really a new entity type but merely a relocation type.
Handle it with a x86_immediate_kind.

This way we don't need a separate frame_entity field in the attribute
struct anymore. We can also use the immediate type to determine whether
we need to assign a fresh stackentity to a node.
This commit does not change the frame_use field, which is now a
duplicate way to indicate whether we need to assign a stackentity. This
currently requires a lot of asserts to ensure the two stay in sync. In
the future I'd like to use frame_use only for the special case when a
32bit or 64bit entity needs to be forced.

It is now required to set a (relocation) kind when entity != NULL.
Adds IMM_ADDR, PICBASE_REL kinds to differentiate between absolute
addresses and picbase relative addresses. Maintain the no_picbase_adjust
flag in parallel for testing. We should be able to remove it in a
subsequent commit.