RFR: 8302462: [REDO] 8297487: G1 Remark: no need to keep alive oop constants of nmethods on stack

[Richard Reingruber]

This is a REDO of JDK-8297487 after the issues that cause the BACKOUT were resolved with [JDK-8300915](https://bugs.openjdk.org/browse/JDK-8300915)

Testing:
* The reproducer `test/langtools:tier1` was executed without crash for 12h.
* Multiple iterations of jtreg tests tiers 1-4  on standard platforms and also on ppc64le.

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Commit messages:
 - REDO JDK-8297487

Changes: https://git.openjdk.org/jdk/pull/12561/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12561&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302462
  Stats: 19 lines in 2 files changed: 3 ins; 13 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12561.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12561/head:pull/12561

PR: https://git.openjdk.org/jdk/pull/12561