RFR: 8270865: Print process ID with -Xlog:os

[Johan Sjölen]

Adds logging of the process ID fairly early in the process (right after `HOTSPOT_VM_INIT_END();`) as a quality of life improvement.

Citing the ticket:

> It's useful to add the PID into a log file. For example, we can use it to find the corresponding hs_err_<pid>.log

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Commit messages:
 - Log the PID of the initialized VM

Changes: https://git.openjdk.org/jdk/pull/13015/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13015&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8270865
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13015.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13015/head:pull/13015

PR: https://git.openjdk.org/jdk/pull/13015