[8u20] RFR (S): 8043206: Fix signed vs. unsigned comparison warning in copy_sparc.hpp

[Mikael Vidstedt]

Please review this 8u20 backport request. The fix was pushed to jdk9 a 
week ago and has not shown any problems.

The change from jdk9 applies to jdk8u/hs-dev without conflicts.

Webrev: 
http://cr.openjdk.java.net/~mikael/webrevs/8043206-8u/webrev.00/webrev/
Bug: https://bugs.openjdk.java.net/browse/JDK-8043206
jdk9 change: 
http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/539e0059f825

Thanks,
Mikael