Add debuginfod support for objdump -S

Add debuginfod support for objdump -S

Currently objdump -S is not able to make use files downloaded from debuginfod.
This is due to bfd_find_nearest_line_discriminator being unable to locate any
separate debuginfo files in the debuginfod cache. Additionally objdump lacked
a call to debuginfod_find_source in order to download missing source files.

Fix this by using bfd_find_nearest_line_with_alt instead of
bfd_find_nearest_line_discriminator. Also add a call to
debuginfod_find_source in order to download missing source files.

Co-authored-by: Nick Clifton <nickc@redhat.com>