RFR(xxs, jdk10): 8171508: os::jvm_path -XXaltjvm processing error after 8066474

[Thomas Stüfe]

Hi all,

may I please have a review for this tiny fix. 8066474 removed the <arch>
directory from the images and since then -XXaltjvm was slightly broken.
When handling XXaltjvm, os::jvm_path() examines the path of the libjvm.so
to check if it is part of what it considers a standard JDK by traversing a
number of slashes up the path and looking for "/jre/lib". That number of
slashes was off since 8066474.

webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8171508-os_jvm_path_xxaltjvm_processing_error_after_8066474/jdk10-webrev.00/webrev/index.html

Bug: https://bugs.openjdk.java.net/browse/JDK-8171508

Note that this only affects cases where the alternate libjvm.so is part of
a full jdk, so it does not affect the gtestLauncher.

Thanks & Regards, Thomas