If I just change a .cpp file in the hotspot repository, the build also builds this (and insists on telling me about it).

philli% make all
Building target 'all' in configuration 'linux-x86_64-normal-server-fastdebug'
All done.
Creating java.base.jmod
Creating java.logging.jmod
Creating java.datatransfer.jmod
Creating java.annotations.common.jmod
Creating java.xml.jmod
Creating java.scripting.jmod
Creating java.compiler.jmod
Creating java.httpclient.jmod
Creating java.instrument.jmod
Creating java.security.sasl.jmod
Creating java.xml.crypto.jmod
Creating jdk.httpserver.jmod
Creating java.smartcardio.jmod
Creating jdk.jvmstat.jmod
Creating jdk.charsets.jmod
Creating jdk.compiler.jmod
Creating jdk.crypto.ec.jmod
Creating jdk.dynalink.jmod
Creating jdk.internal.le.jmod
Creating jdk.internal.opt.jmod
Creating jdk.jartool.jmod
Creating jdk.javadoc.jmod
Creating jdk.jdeps.jmod
Creating jdk.jdwp.agent.jmod
Creating jdk.jfr.jmod
Creating jdk.localedata.jmod
Creating jdk.management.resource.jmod
Creating jdk.net.jmod
Creating jdk.pack200.jmod
Creating jdk.scripting.nashorn.jmod
Creating jdk.sctp.jmod
Creating jdk.vm.cds.jmod
Creating jdk.vm.ci.jmod
Creating jdk.xml.dom.jmod
Creating jdk.zipfs.jmod
Creating java.prefs.jmod
Creating java.compact1.jmod
Creating java.rmi.jmod
Creating java.sql.jmod
Creating java.naming.jmod
Creating jdk.attach.jmod
Creating jdk.crypto.pkcs11.jmod
Creating jdk.jlink.jmod
Creating jdk.jvmstat.rmi.jmod
Creating java.desktop.jmod
Creating java.compact2.jmod
Creating java.management.jmod
Creating java.security.jgss.jmod
Creating java.sql.rowset.jmod
Creating java.transaction.jmod
Creating jdk.jcmd.jmod
Creating jdk.jdi.jmod
Creating jdk.management.jmod
Creating jdk.jsobject.jmod
Creating jdk.naming.dns.jmod
Creating jdk.naming.rmi.jmod
Creating jdk.security.jgss.jmod
Creating jdk.scripting.nashorn.shell.jmod
Creating jdk.security.auth.jmod
Creating jdk.snmp.jmod
Creating java.activation.jmod
Creating java.compact3.jmod
Creating java.corba.jmod
Creating jdk.accessibility.jmod
Creating jdk.hotspot.agent.jmod
Creating jdk.jconsole.jmod
Creating jdk.jshell.jmod
Creating jdk.management.cmm.jmod
Creating jdk.management.jfr.jmod
Creating jdk.policytool.jmod
Creating jdk.rmic.jmod
Creating java.se.jmod
Creating java.xml.bind.jmod
Creating jdk.xml.bind.jmod
Creating java.xml.ws.jmod
Creating java.se.ee.jmod
Creating jdk.xml.ws.jmod
Creating jdk server jimage
Creating jdk jimage
Creating jre jimage
WARNING: Installer build disabled
philli% Finished building target 'all' in configuration 'linux-x86_64-normal-server-fastdebug'

If all I'm building is a new object for libjvm.so, this shouldn't be necessary. My machine is too slow for this!