[ELF] --compress-debug-sections=zstd: replace ZSTD_c_nbWorkers parallelism with multi-frame parallelism

https://reviews.llvm.org/D133679 utilizes zstd's multithread API to
create one single frame. This provides a higher compression ratio but is
significantly slower than concatenating multiple frames.

With manual parallelism, it is easier to parallelize memcpy in
OutputSection::writeTo for parallel memcpy.

In addition, as the individual allocated decompression buffers are much
smaller, we can make a wild guess (compressed_size/4) without worrying
about a resize (due to wrong guess) would waste memory.

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