RFR: 7903720: Change layout of jextract image [v4]

[Jorn Vernee]

> See JBS issue for problem description.
> 
> This patch changes the layout of the jextract image we produce. The new image has the 2 launcher scripts in the root of the image, with a nested `runtime` folder containing the jlink runtime image.
> 
> I've added both launcher scripts to the repo under the `src/main` directory, and they are copied to the image during the build. The script is somewhat simplified by removing a blank variable that was not being used. I also took the opportunity to address an issue with the windows launcher script turning off `echo`. Instead it now avoids output using the `@` prefix for each command in the script.
> 
> I've tested that the scripts work (using `jextract --help`) on both Linux and Windows.

Jorn Vernee has updated the pull request incrementally with four additional commits since the last revision:

 - set mac version to x64 pt2
 - set mac version to x64
 - only set file permissions if supported
 - pass JEXTRACT_JAVA_OPTIONS to launcher

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/241/files
  - new: https://git.openjdk.org/jextract/pull/241/files/89b8e2bd..a2768cfb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=241&range=03
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=241&range=02-03

  Stats: 13 lines in 4 files changed: 2 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jextract/pull/241.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/241/head:pull/241

PR: https://git.openjdk.org/jextract/pull/241