A64: Update DoAllocate() to use a W register for the size

Inputs should always be 32bit

BUG=none
R=m.m.capewell@googlemail.com, ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/167123004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19426 ce2b1a6d-e550-0410-aec6-3dcde31c8c00