RFR: 8304591: UnixPath.stringValue shouldn't be volatile

[Sergey Tsypanov]

`UnixPath.stringValue` can be computed by multiple threads safely as the actual value is the same, so the race is benign in this case.

-------------

Commit messages:
 - Fix build
 - UnixPath.stringValue shouldn't be volatile

Changes: https://git.openjdk.org/jdk/pull/13115/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304591
  Stats: 9 lines in 1 file changed: 3 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13115.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13115/head:pull/13115

PR: https://git.openjdk.org/jdk/pull/13115