I believe GraalVM is implemented in Java, so it doesn’t get built from source, what you need is to build from source another JVM with JVMCI support, which is then used to byte-compile the GraalVM java source into java bytecode, and then you use that other JVM to run GraalVM.
Basically GraalVM is a Java application.
Though I think you can then use GraalVM running in another JVM to native-image compile GraalVM itself.