RFR: 8315891: java/foreign/TestLinker.java failed with "error occurred while instantiating class TestLinker: null"

[Maurizio Cimadamore]

This PR adds a privileged block surrounding the request to load the fallback linker library in LibFallback.
The lack of this block is causing test failures when platforms using the fallback linker are tested using a security manager.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/15633/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15633&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315891
  Stats: 13 lines in 1 file changed: 6 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/15633.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15633/head:pull/15633

PR: https://git.openjdk.org/jdk/pull/15633