A64: Fix register usage in LCodeGen::ArgumentsLength

The result should be just stored in a W register

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

Review URL: https://codereview.chromium.org/170363002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00