Please update libbacktrace for better DWARF5 support

https://github.com/ianlancetaylor/libbacktrace now includes updates that improve DWARF5 handling. It would be nice if `rust-lang/libbacktrace` and `backtrace-sys` could benefit from this.

xref: https://bugzilla.redhat.com/show_bug.cgi?id=1929734