libproc: Improve .gnu_debuglink support.

As of r278658 libproc looks for debug files under /usr/lib/debug and will
use them if available. This change fleshes out that support a bit further:

• Check for a .gnu_debuglink section and use the file name specified there if one is present.
• Validate external debug files with the CRC in the .gnu_debuglink section so as to avoid using stale or corrupt debug files.
• Search for debug files in the directory containing the referencing object or in the .debug subdirectory, as GDB does.