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

[Aleksey Shipilev]

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)

-- 
Thanks,
-Aleksey