Parallelise JVM backend - Scala 2 port

[WojciechMazur]

This PR ports scala/scala#6124 introduces backend parallelism, on top of the previously ported changes introduced in #15322
Adds Scala2 flags:

• -Yjar-compression-level:N
• -Ybackend-parallelism:N
• -Ybackend-worker-queue:N

[dottybot]

performance test scheduled: 1 job(s) in queue, 1 running.

[dottybot]

performance test scheduled: 1 job(s) in queue, 1 running.

[dottybot]

Performance test finished successfully:

Visit https://dotty-bench.epfl.ch/15392/ to see the changes.

Benchmarks is based on merging with main (76a0b29)

[sjrd]

I can't say I have carefully reviewed everything, but it seems to be a reasonable enough port of the Scala 2 PR. I only have a few comments to make it a bit safer to evolve later.

[He-Pin]

@WojciechMazur Will this feature be backported to the 3.3.2 release?

[WojciechMazur]

Will this feature be backported to the 3.3.2 release?

I don't think it will or should be it backported. In the Scala 2 JVM backend contained additional postprocessing (optimizer, inliner) of the Bytecode which actually benefited from fact of its paralysation. However, currently in Scala 3 we would not see any reasonable performance gains as we would be able to only write generated classfiles in parallel - typically we would be limited by I/O, and not the CPU.
This might change when we'd add the bytecode optimizer, but the effort in this area has just began, so it probably would not target 3.3.x soon or at all.