RFR: 8281335: Allow a library already loaded via System::loadLibrary to be loaded as a raw library

[Mandy Chung]

This patch removes the restriction in the raw library loading mechanism that does not allow mix-n-match of loading a library as a JNI library and as a raw library.

The raw library loading mechanism is designed for panama to load native library essentially equivalent to dlopen/dlclose calls independent of JNI library loading. If a native library is loaded as a JNI library and a raw library, it will get different NativeLibrary instances. When a class loader is being unloaded, JNI_Unload will be invoked but the native library may not be unloaded until NativeLibrary::unload is explicitly called for the raw library.

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Commit messages:
 - Allow raw libraries to be loaded even if it's loaded via System::loadLibrary

Changes: https://git.openjdk.java.net/jdk/pull/7435/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7435&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281335
  Stats: 324 lines in 4 files changed: 191 ins; 115 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7435.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7435/head:pull/7435

PR: https://git.openjdk.java.net/jdk/pull/7435