RFR: 8271951: Consolidate preserved marks overflow stack in SerialGC

[Roman Kennke]

In SerialGC we put preserved marks in space that we may have in the young gen, and failing that, we push it to an overflow stack. The overflow stack is actually two stacks, and replicate behavior of the in-place preserved marks. These can be consolidated.

Testing:
 - [ ] tier1
 - [ ] tier2
 - [ ] hotspot_gc

-------------

Commit messages:
 - 8271951: Consolidate preserved marks overflow stack in SerialGC

Changes: https://git.openjdk.java.net/jdk/pull/5022/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5022&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271951
  Stats: 23 lines in 3 files changed: 2 ins; 10 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5022.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5022/head:pull/5022

PR: https://git.openjdk.java.net/jdk/pull/5022