bfd: Add bfd_find_nearest_line_with_alt

bfd_find_nearest_line_with_alt functions like bfd_find_nearest_line with
the addition of a parameter for specifying the filename of a supplementary
debug file such as one referenced by .gnu_debugaltlink or .debug_sup.

This patch focuses on implementing bfd_find_nearest_line_with_alt
support for ELF/DWARF2 .gnu_debugaltlink. For other targets this
function simply sets the invalid_operation bfd_error.