jcstress: Update buffer tests for JDK-8318966

JDK-8320247 changed the requirement for heap buffers to be aligned by 8 bytes. Notably Buffer.alignmentOffset(0, 8) would now throw the exception for non-direct buffers. This means lots of jcstress tests are now failing:

```
  Messages:
    Check test failed
    java.lang.ExceptionInInitializerError
     at org.openjdk.jcstress.tests.accessAtomic.varHandles.byteArray.big.acqrel.ShortTest_jcstress.jcstress_sanityCheck_API(ShortTest_jcstress.java:80)
     at org.openjdk.jcstress.tests.accessAtomic.varHandles.byteArray.big.acqrel.ShortTest_jcstress.sanityCheck(ShortTest_jcstress.java:29)
     at org.openjdk.jcstress.infra.runners.Runner.run(Runner.java:64)
     at org.openjdk.jcstress.ForkedMain.main(ForkedMain.java:79)
    Caused by: java.lang.UnsupportedOperationException: Unit size unsupported for non-direct buffers: 8
     at java.base/java.nio.ByteBuffer.alignmentOffset(ByteBuffer.java:2241)
     at org.openjdk.jcstress.tests.accessAtomic.varHandles.byteArray.big.acqrel.ShortTest.<clinit>(ShortTest.java:47)
     ... 4 more


```