RFR: 8340081: Test java/foreign/TestLinker.java failed failed: missing permission java.lang.foreign.native.threshold.power.fill

[Per Minborg]

This PR fixes a regression introduced by https://github.com/openjdk/jdk/pull/20848

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Commit messages:
 - Use privilegedGetProperty

Changes: https://git.openjdk.org/jdk/pull/20983/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20983&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340081
  Stats: 10 lines in 1 file changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20983.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20983/head:pull/20983

PR: https://git.openjdk.org/jdk/pull/20983