RFR: 8293499: Provide jmod compression level option

[Claes Redestad]

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.

Could we word this so that the accepted values is compression-dependent? The value appears to be ignored for `--compress=0|1` (should it throw if specified or not specified to, say, 0?). We might also add other compressions in the future which allows a wider or narrower range of compression levels (zstd uses 0 - 22).

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