* Add test case for line-tables-only

* Support line-tables-only when using libbacktrace

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
```

Co-authored-by: Tomasz Miąsko <[email protected]>

* Update libbacktrace to latest master

* Temporarily switch to t6/libbacktrace which has 5d4a5ff0d17040b5dd5dfaf2c3694b9a7b67b85d

* Define HAVE_MACH_O_DYLD_H on macOS

* Define HAVE_KERN_PROC on FreeBSD to fix rust-lang/rust#54434

For the same reason define HAVE_KERN_PROC_ARGS on NetBSD.

This depends on having [1] available in libbacktrace.

[1] ianlancetaylor/libbacktrace@0f06cda

* Add dragonfly too