[foreign-memaccess+abi] 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.

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Commit messages:
 - Read environment variable using doPrivileged

Changes: https://git.openjdk.org/panama-foreign/pull/865/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=865&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314260
  Stats: 11 lines in 1 file changed: 10 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/865.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/865/head:pull/865

PR: https://git.openjdk.org/panama-foreign/pull/865