First stage of implementing an out-of-line constant pool on Arm.  This CL
frees up register r7 for use as a constant pool pointer in later stages.

BUG=
R=ulan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21063002

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16898 ce2b1a6d-e550-0410-aec6-3dcde31c8c00