RFR: 8309216: Cast from jchar* to char* in test java/io/GetXSpace.java

[Brian Burkhalter]

For Unix, copy the `jchar`s into an allocated `char` array.

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Commit messages:
 - 8309216: Cast from jchar* to char* in test java/io/GetXSpace.java

Changes: https://git.openjdk.org/jdk/pull/14276/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14276&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309216
  Stats: 23 lines in 1 file changed: 19 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14276.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14276/head:pull/14276

PR: https://git.openjdk.org/jdk/pull/14276