RFR: 8314260: Unable to load system libraries on Windows when using a SecurityManager [v2]

[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.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Add a SecurityManager for TestLinker

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15564/files
  - new: https://git.openjdk.org/jdk/pull/15564/files/6b8e4299..74c3d2c9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15564&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15564&range=00-01

  Stats: 9 lines in 2 files changed: 8 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