RFR: 7903720: Change layout of jextract image [v2]

[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 one additional commit since the last revision:

  set file permissions to executable for jextract script

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/241/files
  - new: https://git.openjdk.org/jextract/pull/241/files/75998f5d..712670c3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=241&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=241&range=00-01

  Stats: 0 lines in 1 file changed: 0 ins; 0 del; 0 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