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

[Erik Joelsson]

On Fri, 20 Oct 2023 14:25:30 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> 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.

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/16288#pullrequestreview-1690600506