RFR [11u]: Fix x86_32 build failures after CAS barriers backport

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/11-fix-cas/webrev.01/

CAS backport broke x86_32 builds, because Shenandoah identifiers are not accessible when Shenandoah
compilation is disabled. We can work that around until we backport more stuff.

Testing: Linux (x86_32, x86_64) builds; tier1_gc_shenandoah

-- 
Thanks,
-Aleksey