RFR: 8357550: GenShen crashes during freeze: assert(!chunk->requires_barriers()) failed

[William Kemper]

Shenandoah captures a preliminary top-at-mark-start (TAMS) concurrently. Once concurrent marking has begun, any object beneath TAMS requires a barrier when accessed. The implementation of `ShenandoahHeap::requires_barrier` does not distinguish between _old_ and _young_ marking. This could lead to a race between freezing the stack and capturing TAMS:

1. Collector is marking old
2. Allocate stack chunk in young
3. Barrier is not needed, begin fast freeze path
4. Collector concurrently updates TAMS (young marking will start once init-mark safepoint is reached)
5. ShenandoahHeap now thinks the barrier is needed (because TAMS changed, and we are marking old)
6. Fast freeze asserts out

This fix has the generational mode check the appropriate generation and its marking state. It also enforces barriers for stackChunks in the old generation (for card marking).

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

Commit messages:
 - override requires_barriers for generational mode

Changes: https://git.openjdk.org/jdk/pull/25905/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25905&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357550
  Stats: 25 lines in 2 files changed: 25 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25905.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25905/head:pull/25905

PR: https://git.openjdk.org/jdk/pull/25905