[ELF] Parallelize --compress-debug-sections=zstd

See D117853: compressing debug sections is a bottleneck and therefore it
has a large value parallizing the step.

zstd provides multi-threading API and the output is deterministic even with
different numbers of threads (see https://github.com/facebook/zstd/issues/2238).
Therefore we can leverage it instead of using the pigz-style sharding approach.

Also, switch to the default compression level 3. The current level 5
is significantly slower without providing justifying size benefit.

```
  'dash b.sh 1' ran
    1.05 ± 0.01 times faster than 'dash b.sh 3'
    1.18 ± 0.01 times faster than 'dash b.sh 4'
    1.29 ± 0.02 times faster than 'dash b.sh 5'

level=1 size: 358946945
level=3 size: 309002145
level=4 size: 307693204
level=5 size: 297828315
```

Reviewed By: andrewng, peter.smith

Differential Revision: https://reviews.llvm.org/D133679