RFR (XXS) 8231201: hs_err should print coalesced safepoint operations in Events section

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8231201
  https://cr.openjdk.java.net/~shade/8231201/webrev.01/

Little debugging aid here. See examples in the bug. The patch is one-liner, but webrev is provided
to give reviewers a better context, for example to compare with other EventMark and
log_debug(vmthread) statements.

Testing: eyeballing hs_errs, jdk-submit (running)

-- 
Thanks,
-Aleksey