Fallback to using file offsets if no symbol is found, like we do if the
object could not be opened.

This makes backtraces usable even if objects in the trace are stripped,
since the trace can be symbolized by post-processing it with a tool like
asan_symbolize.py.

Note that this is not currently compatible with SymbolizeCallback as
this overwrites the filename in the buffer. The behavior is unchanged
in that case.

Closes: #514