elf: Support separate debug info via build-id

This adds support for looking in separate files for debug info as per:
http://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

We only support build-id which is newer and more efficient.
Verified that this works on Fedora 18.
Merge pull request #13 from alexlarsson/separate-debug

elf: Support separate debug info via build-id