RFR: 8352914: Shenandoah: Change definition of ShenandoahSharedValue to int32_t to leverage platform atomics

[Ben Taylor]

The `STATIC_ASSERT` below this typedef appears to be out of date. 

The barriers check thread local copy of gc state, which is stored in `ShenandoahThreadLocalData::_gc_state`  and is type `char`, so the size requirement described by the assert is maintained even after this change.

Change passes all tier1 tests locally when run with Shenandoah GC.

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

Commit messages:
 - 8352914: Shenandoah: Change definition of ShenandoahSharedValue to int32_t to leverage platform atomics

Changes: https://git.openjdk.org/jdk/pull/28681/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28681&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352914
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28681.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28681/head:pull/28681

PR: https://git.openjdk.org/jdk/pull/28681