RFR (S): 8144663: Invalid constraints in memset_with_concurrent_readers_sparc.cpp inline assembly

[Mikael Vidstedt]

Please review this change which fixes the constraints used in the inline 
assembly snippets in memset_with_concurrent_readers_sparc.cpp.

Bug: https://bugs.openjdk.java.net/browse/JDK-8144663
Webrev: http://cr.openjdk.java.net/~mikael/webrevs/8144663/webrev.00/webrev/

The bug has more information, but in short the "+" needs to be the first 
character in the constraints string, and when used the corresponding 
constraint needs to be listed in the output constraints.

I also took the liberty of clarifying one of the comments a bit, 
hopefully it's now ever so slightly more clear that the registers listed 
in the output constraints, while indeed being outputs, are only used as 
temporaries/are overwritten.

Finally, I fixed a couple of comparisons of unsigned vs signed types - 
BytesPerWord is "unfortunately" signed.

Tested using the -XX:+ExecuteInternalVMTests tests.

Cheers,
Mikael