RFR: 8282515: More clean up on NativeLibraries just for JNI library use

[Mandy Chung]

This patch further cleans up NativeLibraries just for JNI library use.  RawNativeLibraries implements its own native load and unload methods.  In addition, this also fixes the implementation of `RawNativeLibraries::load` not to throw UnsatisfiedLinkError if a library cannot be loaded for any reason but instead returns null.

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Commit messages:
 - 8282515: More clean up on NativeLibraries just for JNI library use

Changes: https://git.openjdk.java.net/jdk/pull/7661/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7661&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282515
  Stats: 275 lines in 6 files changed: 175 ins; 28 del; 72 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7661.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7661/head:pull/7661

PR: https://git.openjdk.java.net/jdk/pull/7661