RFR: Assembler write barriers should consistently check for forwarded objects

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/sbs-forwarded-check/webrev.01/

This fixes the bug in write_barrier_impl in x86_64: we should check either for 0, or for
HAS_FORWARDED when deciding to omit the read barrier. C1 write barrier tests HAS_FORWARDED and it
seems most efficient when coalescing is not available. So, the rest homogenizes the check shape
across architectures.

Testing: tier3_gc_shenandoah (x86_64), build (aarch64)

Thanks,
-Aleksey