WAR: Add arm_neon.h prebuilt to external/clang.

Bug: 12979170

TODO: Remove this after we support moving all relevant headers into a single
directory with the appropriate module.map.

Missing header, required by build for aosp and linaro clang 3.4+
Change-Id: Ifd9f454350429431e66c7f1a9d85bdce4f174629