RFR: 8274293: Build failure with Xcode 13.0 due to vfork is deprecated

[xpbob]

remove vfork() on Darwin

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

Commit messages:
 - 8274293: Build failure with Xcode 13.0 due to vfork is deprecated

Changes: https://git.openjdk.java.net/jdk/pull/5686/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5686&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274293
  Stats: 10 lines in 2 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5686.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5686/head:pull/5686

PR: https://git.openjdk.java.net/jdk/pull/5686