newt: Extend link tables symbols

[kasjer]

So far for link time generated tables two symbols
were create for each table:

pkg.link_tables:
    - foo

would generate link_tables.ld.h with following content

        __foo_start__ = .;
        KEEP(*(.foo))
        KEEP(*(SORT(.foo.*)))
        __foo_end__ = .;

Now additional symbol foo will be generated it will make it more straightforward to use link-time tables in source code where table can be access directly
by a name specified in yml

        __foo_start__ = .;
        foo = .;
        KEEP(*(.foo))
        KEEP(*(SORT(.foo.*)))
        __foo_end__ = .;