linker_lists: Allow use in data structures

At present linker lists are designed for use in code. They make use of
statements within expressions ({...}), for example.

It is possible to generate a reference to a linker_list entry that can
be used in data structures, where such features are not permitted. It
requires that the reference first be declared as extern. In other
words the existing macro needs to be split into two parts.

Add new macros to support this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>