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

The map maps global symbols to the respective entities in the segment
types. This should avoid hacks in some frontends, avoids a sepcial map
for compilerlib entities and ensures that frontend and compilerlib
entities are in sync.

This is cleaner than having them being class types with a tf_segment
flag set.

The simple tp_opcode enum is enough, no need to have dispatch tables and
information structs behind each type kind.

We can get a proper mode for pointer-pointer from
get_reference_offset_mode().

Also choose a signed integer type which is more typical for offsets.

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.

It is barely used but cross cuts into most libfirm components and
therefore isn't maintenance free.

The statev framework stays obviously.

For stabs debug info it was used to suppress debug info, so set ir_visibility_private instead.

Avoid using the op_pin_state enum as that contains confusing additional
entries that are not valid on a node. The only remaining state for a
node is pinned/floating which we can simply represent with a 0/1 value.

This adds two builtin kinds, ir_bk_va_start and ir_bk_va_arg. va_arg Builtins
are lowered in the lower_builtins pass with a function provided by the backend,
whereby most architectures can use be_default_lower_va_args. va_start Builtins
are lowered in the backend.

Note: This commit breaks all backends except for IA32. SPARC and AMD64 are
implemented in later commits. The ARM backend previously miscompiled variadic
functions, and will now throw a proper error.