Webrev: http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8240975/webrev.00/
This is a follow-up task for Panama to allow explicit unloading of native library after JDK-8228336. `NativeLibraries` associated with a class loader has the capability to auto unload native libraries when the class loader is reclaimed. This adds a new `NativeLibraries::unload` method that provides the ability to unload a native library on request. This only allows `NativeLibraries` of no relationship with a class loader where auto unloading is disabled.
Thanks Mandy
