[Arm]: Make Assembler::movw only emit a movw instruction.

[Arm]: Make Assembler::movw only emit a movw instruction.

Currently Assembler::movw is really the mov macro instruction, leading to raw
emit calls to generate the real movw instruction. Replace all calls of mow
with the mov macro instruction (which will emit a movw if appropriate) and
make movw always emit movw.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/329233002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22085 ce2b1a6d-e550-0410-aec6-3dcde31c8c00