When doing cross-compilation builds you need to copy hotspot into a host JDK image that mataches your target architecture. At present the copy_jdk (and other) rules use the setting of JDK_IMPORT_PATH, but that setting has a dual role in a full JDK build and in general needs to refer to a JDK that can run on the host doing a build.
JPRT has just been enhanced to add two new environment variables when doing a build:
ALT_JDK_HOST_IMPORT_PATH - JDK that the host can execute
ALT_JDK_TARGET_IMPORT_PATH - JDK for the target architecture
for cross-compile builds the target will be set based on properties defined in the JPRT system. For non-cross-compile ALT_JDK_TARGET_IMPORT_PATH will either not be set (seems to be current case) or else will be the same as ALT_JDK_HOST_IMPORT_PATH which in turn will be the same as JDK_IMPORT_PATH.
This fix will change the JDK copying tasks in the Makefile to use ALT_JDK_TARGET_IMPORT_PATH if it is defined.