[Bug other/82857] libbacktrace: please support binaries stripped with dwz -m (following the .gnu_debugaltlink)

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82857

--- Comment #5 from Tom de Vries <vries at gcc dot gnu.org> ---
Created attachment 44962
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44962&action=edit
WIP patch, handles DW_FORM_GNU_strp_alt

With this patch, the test-case from comment 1 passes:
...
Non-stripped
1
Stripped
0
Stripped with debuglink
1
Stripped with debuglink to compressed with objcopy
1
Stripped with debuglink to compressed with dwz
1
Stripped with debuglink to compressed with dwz -m
1
...

In more detail, we get the backtrace with function name 'main', and the full
path for bt.c:
...
0x400eff main
        /data/gcc_versions/devel/bt.c:20
0x7fe303fcaf49 __libc_start_main
        ../csu/libc-start.c:308
0x400da9 ???
        ../sysdeps/x86_64/start.S:120
0xffffffffffffffff ???
        ???:0
...