RFR: 8314260: Unable to load system libraries on Windows when using a SecurityManager

[Per Minborg]

This PR proposes to read the system environment variable "SystemRoot" using a privileged operation so it will work in the event a default SecurityManager is in place.

As the `SecurityManager` is deprecated for removal, no support methods were added for reading environmental variables.

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Commit messages:
 - Use privilidged access for getenv call

Changes: https://git.openjdk.org/jdk/pull/15564/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15564&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314260
  Stats: 10 lines in 1 file changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15564.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15564/head:pull/15564

PR: https://git.openjdk.org/jdk/pull/15564