RFR: 8317807: JAVA_FLAGS removed from jtreg running in JDK-8317039

[Magnus Ihse Bursie]

Unfortunately, [JDK-8317039](https://bugs.openjdk.org/browse/JDK-8317039) caused a regression. The `$(JAVA)` variable did not just contain a path, which was assumed by the patch, but also the necessary flags in `$(JAVA_FLAGS)`.

Furthermore, the original implementation deviated from how we typically treat JDKs in the build system, that is, we store the root to the JDK as the primary setting, and then we use that as base to get a fixpath'ed executable command line. I've changed the code to more closely align to our existing uses of external JDKs.

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Commit messages:
 - 8317807: JAVA_FLAGS removed from jtreg running in JDK-8317039

Changes: https://git.openjdk.org/jdk/pull/16288/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16288&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317807
  Stats: 14 lines in 4 files changed: 5 ins; 3 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/16288.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16288/head:pull/16288

PR: https://git.openjdk.org/jdk/pull/16288