RFR: 8298471: Parallel: Don't keep alive nmethods in Young GC

[Albert Mingkun Yang]

Small change to skip marking nmethods during scavenging.

Test: hotspot_gc

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Commit messages:
 - pgc-young-keepalive

Changes: https://git.openjdk.org/jdk/pull/11609/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11609&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298471
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11609.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11609/head:pull/11609

PR: https://git.openjdk.org/jdk/pull/11609