ARM: make MacroAssembler::LoadRoot emit exactly one instruction when in predictable code size scope.

BUG=v8:2439
R=danno@chromium.org

Review URL: https://chromiumcodereview.appspot.com/11280276

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00