RFR (S) 8165929: Constify arguments of Copy methods

[coleen.phillimore at oracle.com]

I actually didn't see any calls where const was cast away, and I wasn't 
going to change them anyway.  Most of the calls to the Copy functions 
cast both arguments to HeapWord when there's casting involved.  Anyway, 
this should help for future uses and cleanups. This is very limited and 
hopefully a very boring review.

I wasn't able to test PPC and AARCH64, but tested Oracle's current and 
former supported platforms with this change (note 32 bit arm reverses 
arguments to one of the _Copy functions).   I compiled with Zero but 
can't run Zero for some reason unrelated.

open webrev at http://cr.openjdk.java.net/~coleenp/8165929.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8165929

Thanks,
Coleen