You could already pass compilejdk to jtreg via the options:
`make test-tier1 CONF=linux-riscv64-server-fastdebug JTREG="TIMEOUT_FACTOR=20;OPTIONS=-e:QEMU_LD_PREFIX=/usr/riscv64-linux-gnu/ -compilejdk:/home/rehn/source/jdk/vanilla/build/linux-x86_64-server-release/images/jdk;JAVA_OPTIONS=-XX:+UnlockExperimentalVMOptions -XX:+UnlockDiagnosticVMOptions -XX:+UseZfh;RETAIN=all" JDK_FOR_COMPILE=/home/rehn/source/jdk/vanilla/build/linux-x86_64-server-release/images/jdk`

With this change I can no longer supply compilejdk as an options. (duplicate compilejdk options)
If there is no JDK_FOR_COMPILE set, i.e. default, we should not add it to command line IMHO.

Now I need to have different command-line before and after this changes, hence if I bisect a bug or run in CI I need to know if I'm before or after this commit which is a bit problematic.
And the backports make it even more problematic as an older JDK may need it while a newer don't.

Also JDK_FOR_COMPILE is not documented and I get this warning:

Note: Command line contains non-control variables:
* JDK_FOR_COMPILE=/home/rehn/source/jdk/vanilla/build/linux-x86_64-server-release/images/jdk
Make sure it is not mistyped, and that you intend to override this variable.
'make help' will list known control variables.