Add back a static_cast<int32_t> to assembler-arm.cc to fix WebView build.

The WebView Mac builder is having trouble with resolving set_at_offset with an
intptr_t argument to the correct overload. Add back the static_cast which
was present before r22293 to resolve this.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/398893003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22446 ce2b1a6d-e550-0410-aec6-3dcde31c8c00