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

[Jorn Vernee]

> 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

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

Changes:
  - all: https://git.openjdk.java.net/jextract/pull/9/files
  - new: https://git.openjdk.java.net/jextract/pull/9/files/1b4682d9..d6434bd7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=9&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=9&range=00-01

  Stats: 79 lines in 3 files changed: 62 ins; 5 del; 12 mod
  Patch: https://git.openjdk.java.net/jextract/pull/9.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/9/head:pull/9

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