ARM: use vstm/vldm when possible.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/74193004

Patch from Rodolph Perfetta <rodolph.perfetta@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17854 ce2b1a6d-e550-0410-aec6-3dcde31c8c00