[crac] RFR: 8357641: [CRaC] Support PID change during checkpoint-restore

[Timofei Pushkin]

Adds updates of cached PID on restore.

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Commit messages:
 - JVM/JDK code using getpid() and gettid() returns stale values on GLIBC < 2.25
 - Reset PID in ProcessHandleImpl on restore
 - Replace getpid() use with syscall in os::current_process_id()

Changes: https://git.openjdk.org/crac/pull/229/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=229&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357641
  Stats: 96 lines in 15 files changed: 87 ins; 2 del; 7 mod
  Patch: https://git.openjdk.org/crac/pull/229.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/229/head:pull/229

PR: https://git.openjdk.org/crac/pull/229