Adds support for resolving symbols using external debuginfo files,
which can be retrieved from two locations. First, check the build-id
of the desired binary and look in /usr/lib/debug/.build-id according
to the build-id structure. Second, check the debuglink section of
the desired binary and look in /usr/lib/debug or in the binary's
current directory. These are the rules applied by GDB as well, but
GDB lets the user reconfigure the debug directory path from
/usr/lib/debug to something else; we do not support this.

These changes are based on the following description of how GDB
resolves external debuginfo:

https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html