use memcpy for unaligned neon stores

use memcpy for unaligned neon stores

Advise the compiler that the store is eventually going to a uint8_t
buffer. This helps avoid getting alignment hints which would cause the
memory access to fail.

Originally added as a workaround for clang:
https://bugs.llvm.org//show_bug.cgi?id=24421

Change-Id: Ie9854b777cfb2f4baaee66764f0e51dcb094d51e