mk: Clean libbacktrace w/ gusto

After `make clean' I'm seeing the build break with

```
cp: cannot stat ‘x86_64-unknown-linux-gnu/rt/libbacktrace/.libs/libbacktrace.a’: No such file or directory
```

Deleteing the libbacktrace dir entirely on clean fixes.
auto merge of #12899 : brson/rust/cleanbacktrace, r=alexcrichton

After `make clean` I'm seeing the build break with

```
cp: cannot stat ‘x86_64-unknown-linux-gnu/rt/libbacktrace/.libs/libbacktrace.a’: No such file or directory
```

Deleteing the libbacktrace dir entirely on clean fixes.