RFR: 8317039 Enable specifying the JDK used to run jtreg

[Ludvig Janiuk]

This pull request introduces changes in the build makefiles necessary to enable the user to specify a jdk to use when launching jtreg. It introduces a new configure flag, `--with-jtreg-jdk`, which takes a JDK path similar to the already existing `--with-boot-jdk`.

Before this pull request, boot-jdk was always used to launch jtreg. This pull request decouples the two, but still defaults to boot-jdk if no value is specified.

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Commit messages:
 - 8317039 Enable specifying the JDK used to run jtreg

Changes: https://git.openjdk.org/jdk/pull/16010/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16010&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317039
  Stats: 42 lines in 4 files changed: 40 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16010.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16010/head:pull/16010

PR: https://git.openjdk.org/jdk/pull/16010