Request for review(M): 7020521: arraycopy stubs place prebarriers incorrectly

[Igor Veresov]

I rearranged the pre-barrier placement in arraycopy stubs so that they 
are properly called in case of chained calls. Also refactored the code a 
little bit so that it looks uniform across the platforms and is more 
readable.

Webrev: http://cr.openjdk.java.net/~iveresov/7020521/webrev.00

Thanks,
igor