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

This reverts r21148, it broke tests in debug mode, e.g.
mjsunit/regress/regress-observe-map-cache or mjsunit/debug-stepout-scope-part5.

TBR=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/296823014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21449 ce2b1a6d-e550-0410-aec6-3dcde31c8c00