DW_FORM_GNU_ref_alt and DW_FORM_GNU_strp_alt refer to data stored in an
external ELF file specified by a .gnu_debugaltlink attribute. These
attributes are generated by dwz(1), which extracts DWARF data common
amongst several files and stores it in a single, new file. It leaves
behind these two forms in the original file to point at the new, common
data.

We don't support actually reading the .gnu_debugaltlink file in
addr2line.c (and maybe we don't really need to), but we do need to know
how to read the actual value of these forms so we can skip over the
right number of bytes and not lose track of where we are in the CU.