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

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

Committed: https://code.google.com/p/v8/source/detail?r=22446

[rmcilroy, 2014-07-17 10:14:56]

Benedikt could you please take a look at this CL, thanks.

[Benedikt Meurer, 2014-07-17 10:29:36]

LGTM

[rmcilroy, 2014-07-17 10:35:09]

Committed patchset #1 manually as r22446 (presubmit successful).