Do away with variable length memcpy to Set/Get registers in simulator

About a 32% boost.

Before - 5:31

Richards: 84.5
DeltaBlue: 128
Crypto: 65.3
RayTrace: 203
EarleyBoyer: 149
RegExp: 23.4
Splay: 121
NavierStokes: 98.9
----
Score (version 7): 93.8

After - 4:10

Richards: 107
DeltaBlue: 175
Crypto: 93.9
RayTrace: 258
EarleyBoyer: 186
RegExp: 32.7
Splay: 165
NavierStokes: 124
----
Score (version 7): 124

R=jacob.bramley@arm.com, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/213943002

Patch from Fritz Koenig <frkoenig@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21448 ce2b1a6d-e550-0410-aec6-3dcde31c8c00