RFR: JDK-8305507 Add support for grace period before AbortVMOnSafepointTimeout triggers [v2]

[Wojciech Kudla]

> As described https://bugs.openjdk.org/browse/JDK-8305507 this change adds a grace period before AbortVMOnSafepointTimeout activates to allow for JVM warm-up to conclude

Wojciech Kudla has updated the pull request incrementally with one additional commit since the last revision:

  Simplified getting elapsed time since VM start, added test case for -XX:AbortVMOnSafepointTimeoutDelay

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13386/files
  - new: https://git.openjdk.org/jdk/pull/13386/files/ccd433c6..c8c37aec

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13386&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13386&range=00-01

  Stats: 55 lines in 2 files changed: 51 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13386.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13386/head:pull/13386

PR: https://git.openjdk.org/jdk/pull/13386