RFR: 8166651: OrderAccess::load_acquire &etc should have const parameters

[Kim Barrett]

Please review this change to Atomic::load and OrderAccess::load_acquire
overloads to make their source const qualified, e.g. instead of
"volatile T*" make them "const volatile T*".  This eliminates the need
for casting away const when, for example, applying one of these
operations to a member variable when in a const-qualified method.

There are probably places that previously required casting away const
but now do not. Similarly, there are probably places where values
couldn't be const or member functions couldn't be const qualified, but
now can be. I did a little searching and found a few candidates, but
none that were otherwise trivial to add to this change, so haven't
included any.

This change touches platform-specific code for non-Oracle supported
platforms that I can't test, so I'd like reviews from the respective
platform owners.

Aside regarding aarch64: I don't know why gcc's __atomic_load doesn't
const-qualify the source argument; that seems like a bug.  Or maybe
they are, but not documented that way.  And I wonder why the aarch64
port uses __atomic_load rather than __atomic_load_n.

CR:
https://bugs.openjdk.java.net/browse/JDK-8166651

Webrev:
http://cr.openjdk.java.net/~kbarrett/8166651/hotspot.00

Testing:
JPRT