Simplify the creation of instrinsic/runtime function entities a bit and avoid creating duplicate types.

Only IA32_PIC_NONE and IA32_PIC_MACHO so far, but more will come.

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

Handle this similar to the ia32 backend.

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.

No need to adjust Pop which cannot have frame offset destinations,
determine whether the fixup is necessary by checking the base register
used instead of relying on generic frame offset stuff.

The flag has been superseded by the immediate kind.

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.

We can simply patch the Immediate node attached to the Call, all cases
without an Immediate node need PIC adjustment anyway.