Old debug information in IMPORT_JDK is not removed

If the Makefiles produces zipped debug info (libjvm.diz) when building hotspot and the IMPORT_JDK has unzipped debug info (libjvm.debuginfo), then generic_export will only copy libjvm.diz and do nothing about the already existing libjvm.debuginfo.

The JPRT make target jprt_build_fastdebug will then zip the IMPORT_JDK folder and the zipped bundle end up having different zipped and unzipped debug info! This is both a waste of space (having both zipped and unzipped debug information) as well as confusing, since one cannot use the unzipped debug information, since that debug information corresponds to the IMPORT_JDK's libjvm.so.