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: https://v8.googlecode.com/svn/branches/bleeding_edge@20141 ce2b1a6d-e550-0410-aec6-3dcde31c8c00