RFR: 8319373: Serial: Refactor dirty cards scanning during Young GC [v8]

[Albert Mingkun Yang]

> Reading the new code directly is probably easier. The structure is quite similar to its counterpart in Parallel.
> 
> It's mostly perf-neutral, except when dirty cards are scarce. Using `card_scan.java` in [JDK-8310031](https://bugs.openjdk.org/browse/JDK-8310031), I observed ~40% reduction in young-gc pause time with `stride = 32 * 64`.

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  next-obj-card

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16492/files
  - new: https://git.openjdk.org/jdk/pull/16492/files/41439f4a..e8b8e0ba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16492&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16492&range=06-07

  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/16492.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16492/head:pull/16492

PR: https://git.openjdk.org/jdk/pull/16492