Review Request JDK-8164512: Replace ClassLoader use of finalizer with phantom reference to unload native library

[mandy chung]

Updated webrev:
http://cr.openjdk.java.net/~mchung/jdk10/webrevs/8164512/webrev.01/

JNI FindClass change has been separated from this patch [1]. I made 
further clean up to the NativeLibrary implementation and replaced the 
use of Vector/Stack.  I also added a native test to verify the native 
library can be unloaded and reloaded.

Summary: The patch replaces the ClassLoader use of finalizer with 
phantom reference, specifically Cleaner, for unloading native 
libraries.  It registers the class loader for cleanup only if it's not a 
builtin class loader which will never be unloaded.

thanks
Mandy
[1] 
http://mail.openjdk.java.net/pipermail/core-libs-dev/2017-October/049389.html