RFR: 8328928: Serial: Use IsGCActiveMark instead of AutoModifyRestore in SerialHeap::do_collection

[Guoxiong Li]

Hi all,

This patch uses `IsGCActiveMark` instead of `AutoModifyRestore` in `SerialHeap::do_collection` so that all the GCs use the `IsGCActiveMark` uniformly. The tests `make test-tier1_gc` passed locally.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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

Commit messages:
 - JDK-8328928

Changes: https://git.openjdk.org/jdk/pull/18473/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18473&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328928
  Stats: 3 lines in 1 file changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18473.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18473/head:pull/18473

PR: https://git.openjdk.org/jdk/pull/18473