RFR (S) 8232730: Shenandoah: Traversal should not CAS the roots

[Roman Kennke]

Ok!

Thanks,
Roman

> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8232730
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8232730/webrev.01/
> 
> This is captured by asserts from JDK-8232729 with hotspot_gc_shenandoah on x86_64. See more details
> in the bug. The underlying reason for this failure is trying to CAS the roots that are not aligned
> to the pointer size, notably code roots. Normal concurrent cycle avoids this by updating the roots
> with plain stores, Traversal should do the same.
> 
> Testing: {x86_64, x86_32} hotspot_gc_shenandoah; tier1 with Shenandoah (running)
>