RFR: 8274788: Support archived heap objects in ParallelGC

[Calvin Cheung]

With this change when the UseParallelGC is enabled, the CDS archived heap objects could be loaded
into the PSOldGen during VM bootstrap. This improves startup time since the module graph could be
loaded from the archive.


$ perf stat -r 40 -XX:+UseParallelGC -version

before: 0.069244290 seconds time elapsed            ( +-  0.49% )
after:    0.058555167 seconds time elapsed          ( +-  0.49% )

Passed CI tiers 1 - 4 tests.

Thanks @tschatzl for his help on GC changes.

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Commit messages:
 - 8274788: Support archived heap objects in ParallelGC

Changes: https://git.openjdk.java.net/jdk/pull/7655/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7655&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274788
  Stats: 156 lines in 7 files changed: 155 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7655.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7655/head:pull/7655

PR: https://git.openjdk.java.net/jdk/pull/7655