#487 support separate debug files in tools that parse debug info

[Ed Maste]

Binaries and libraries may use the .gnu_debuglink functionality to store debug information in a separate debug file. Tools that parse debug information (like addr2line) should follow the debuglink automatically.

GNU binutils 2.17.50 addr2line:

% addr2line -e /bin/ls 0x4023d0         
??:0

GNU binutils 2.25 addr2line:

% /usr/local/bin/addr2line -e /bin/ls 0x4023d0       
/tank/emaste/src/git-stable-10/bin/ls/ls.c:155

elftoolchain addr2line:

% /tank/emaste/src/elftoolchain/addr2line/addr2line -e /bin/ls 0x4023d0
??:0