RFR: 7903136: Don't require jdk18_home to be set in jextract gradle build [v2]

[Jorn Vernee]

On Tue, 29 Mar 2022 18:18:42 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Drop the requirement to set `jdk18_home` in the gradle build, and rely instead on `JAVA_HOME` being set to JDK 18.
>> 
>> I've also updated the build instructions. The `-Pjtreg_home` flag seems to have been dropped by accident from the test command. I've re-added it. Also renamed `libclang_dir` -> `llvm_dir` which was missed in an earlier change.
>
> Jorn Vernee has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Add automatic JDK download
>  - Have -Pjdk flag to specify jdk manually, and allow jdks with higher version as well

I've updated the PR and added an automatic download step that downloads JDK 18 from jdk.java.net when `-Pjdk` is not specified.

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PR: https://git.openjdk.java.net/jextract/pull/9