Use constant length for memcpy on A64 simulator.
Compiler can't optimize away variable length memcpy.
About a 18% boost.
Before - 5:40
Richards: 75.2
DeltaBlue: 105
Crypto: 64.0
RayTrace: 188
EarleyBoyer: 146
RegExp: 23.8
Splay: 96.2
NavierStokes: 91.9
----
Score (version 7): 85.7
After - 4:39
Richards: 90.8
DeltaBlue: 134
Crypto: 81.7
RayTrace: 227
EarleyBoyer: 177
RegExp: 29.7
Splay: 126
NavierStokes: 108
----
Score (version 7): 106
R=rodolph.perfetta@gmail.com, svenpanne@chromium.org
Review URL: https://codereview.chromium.org/
203263017
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20141
ce2b1a6d-e550-0410-aec6-
3dcde31c8c00