Support symbol+offset lookup in addr2line

The Linux kernel usually ouputs symbol+offset instead of plain code
addresses these days, to avoid leaking ASLR secrets and to handle
dynamically loaded modules.

Converting those with addr2line is somewhat involved: it requires
looking up the symbol first using nm and then manually compute the
offset, and then pass it to addr2line.

This patch implements the necessary steps directly in addr2line,
by looking up the symbol (with demangling if needed) and computing
the offset.

It's possible that a symbol is ambigious with a hex number. In this
case it uses the symbol lookup if the string contains a +. When it isn't
ambigious the + is optional.