RFR: 8293499: Provide jmod compression level option

[Alan Bateman]

On Thu, 8 Sep 2022 07:57:36 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> I have been looking into `make clean-images images` performance, and realized jmod keeps compressing files with default compression level. Tuning that toward lighter compression levels improves build performance considerably, without a heavy loss in *.jmod sizes. 
> 
> This PR allows JMOD to select the compression level. Follow-ups would use this in the build system, see #10214.
> 
> This change nominally requires CSR, but I would like to gauge the reaction to this patch first, before submitting a formal CSR. The interesting asymmetry against `jlink` is: `jlink` provides `--compress` option that only takes `2` for "ZIP compression". I would argue it would be beneficial to add `--compression-level` to `jlink` as well, so to select the compression level there too.

I went through the changes and I don't see anything obviously wrong. The command line option doesn't look out of place although as Mandy points us, the format is not documented so there is flexibility to change it in the future. If some future format doesn't support something like compression level then that option could just emit a warning and do nothing.

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PR: https://git.openjdk.org/jdk/pull/10213