RFR: 8274235: -Xshare:dump should not call vm_direct_exit

[Calvin Cheung]

Just changing the `vm_direct_exit` calls during CDS dump time to `os::_exit`.

Passed tiers 1 and 2 tests on Oracle supported platforms.

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Commit messages:
 - 8274235: -Xshare:dump should not call vm_direct_exit

Changes: https://git.openjdk.org/jdk/pull/9421/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9421&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8274235
  Stats: 9 lines in 3 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/9421.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9421/head:pull/9421

PR: https://git.openjdk.org/jdk/pull/9421