RFR: 8317039 Enable specifying the JDK used to run jtreg [v2]

[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.

Ludvig Janiuk has updated the pull request incrementally with one additional commit since the last revision:

  AC_MSG fixups

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16010/files
  - new: https://git.openjdk.org/jdk/pull/16010/files/c9f7dd25..24bfd496

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16010&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16010&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 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