[jdk21u] RFR: 8307766: Linux: Provide the option to override the timer slack

[Aleksey Shipilev]

Clean backport of experimental feature for in-field use.

Additional testing:
 - [ ] Linux x86_64 fastdebug, new test
 - [ ] Linux x86_64 fastdebug, `tier1`
 - [ ] Linux x86_64 fastdebug, `tier2`
 - [ ] Linux x86_64 fastdebug, `tier3`

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Commit messages:
 - Backport 7173c3009e0999f13eaa4bee5eedee7326f9d124

Changes: https://git.openjdk.org/jdk21u/pull/21/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=21&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307766
  Stats: 176 lines in 3 files changed: 175 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u/pull/21.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/21/head:pull/21

PR: https://git.openjdk.org/jdk21u/pull/21