AArch64: Insufficient memory barriers in shadow region algorithm

I occasionally see this test crash on AArch64 with the following assertion failure:

# Internal Error (/home/ent-user/ci-scripts/jdk_src/src/hotspot/share/oops/compressedOops.inline.hpp:84), pid=85650, tid=85656
# assert(!is_null(v)) failed: narrow klass value can never be zero

See the attached full crash log.

Looks like there's a missing store-store barrier in parallal GC MoveAndUpdateShadowClosure::complete_region, as the stores from set_shadow_region() and mark_filled() can be reordered on a non-TSO processor.

I'll do some more testing and send out a patch.