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

[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.

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Commit messages:
 - libclang_dir -> llvm_dir in instructions
 - Use java.home instead of JAVA_HOME
 - reorder build instructions
 - Add missing jtreg_home property to test command
 - Use JDK 18 from Java home, and add a JDK version check

Changes: https://git.openjdk.java.net/jextract/pull/9/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=9&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903136
  Stats: 43 lines in 2 files changed: 35 ins; 3 del; 5 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