RFR: 8189871: Refactor GC barriers to use declarative semantics

[Kim Barrett]

> On Nov 21, 2017, at 2:07 AM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
> Noticed another issue while looking for something else.

And another one.

The modified unsafe.cpp doesn't use field_offset_(to|from)_byte_offset
in places where Access is being used. Those functions and their use
is intentional documentation and abstraction, despite their
implementations presently being nops. Where Access is being used, the
offset is now just quietly assumed to be in bytes, ignoring that
abstraction. I think the use of those functions ought to be retained.