Is your feature request related to a problem? Please describe.
GraalVM currently doesn't build if the base JDK doesn't include jmods. For mandrel in particular, which does no jlinking, this must be optional. For GraalVM it should link from the run-time image instead (for JDK modules), but this needs to be investigated.

Describe the solution you'd like
Allow mandrel builds to succeed with/without jmods being present in the base JDK.

Describe alternatives you've considered
Require the jmods to be present. Yet, it would be nice to not need this. For example for Temurin 24, JEP 493 is enabled and jmods are not being provided (unless there is a strong reason to).

Additional context
Example failing Windows build:

Compiling com.oracle.truffle.compiler with javac-daemon(JDK 24)... [D:\a\mandrel\mandrel\mandrel\truffle\mxbuild\jdk24\com.oracle.truffle.compiler\bin\com\oracle\truffle\compiler\ConstantFieldInfo.class does not exist]
Archiving WORD... [dependency org.graalvm.word updated]
Missing directory containing JMOD files: D:\a\mandrel\mandrel\openjdk\jmods
Building Java module org.graalvm.word (word.jar) from WORD

See: https://github.com/graalvm/mandrel/actions/runs/11944256969/job/33294889211#step:8:358

Enhancement on the Temurin side which enables JEP 493:
adoptium/temurin-build#4035