Support line-tables-only when using libbacktrace

[tmiasko]

Previously when backtrace_pcinfo succeeded, but failed to obtain a
filename or a function name, the line number would be ignored. Instead,
when successful combine all available information.

For example, when using clang -g1 or -gline-tables-only before:

   1: baz
   2: bar
   3: foo

and after:

   1: baz
             at src/callback.c:5
   2: bar
             at src/callback.c:9
   3: foo
             at src/callback.c:13

Note that the difference is probably not observable on gcc which includes the
relevant debug info on level 1, nor in rustc where the differences between
debug levels 1 and 2 are currently rather limited.

[alexcrichton]

Thanks for this!