RFR: 7903197: add a build step to create jmod file for jextract module

[Athijegannathan Sundararajan]

jmod file created for jextract module and that is used to build jextract app image as well as test image.

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Commit messages:
 - 7903197: add a build step to create jmod file for jextract module

Changes: https://git.openjdk.java.net/jextract/pull/34/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=34&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903197
  Stats: 95 lines in 1 file changed: 48 ins; 41 del; 6 mod
  Patch: https://git.openjdk.java.net/jextract/pull/34.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/34/head:pull/34

PR: https://git.openjdk.java.net/jextract/pull/34