RFR: 8293499: Provide jmod --compress option [v6]

[Aleksey Shipilev]

> 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.
> 
> The interesting asymmetry against `jlink` is: `jlink` provides `--compress` option that only takes `2` for "ZIP compression". (Separately, we could argue if it would be beneficial to extend `--compress` to `jlink` as well, so to select the compression level there too.)

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Mention the default level

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10213/files
  - new: https://git.openjdk.org/jdk/pull/10213/files/092b0ffa..2729dfb4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10213&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10213&range=04-05

  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10213.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10213/head:pull/10213

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