the neon code made several assumptions which were broken by a recent
change: https://review.webmproject.org/2676
update the code with new assumptions and guard them with a compile time
assert
Change-Id: I32a8378030759966068f34618d7b4b1b02e101a0
it's difficult to mux the *_offsets.c files because of header conflicts.
make three instead, name them consistently and partititon the contents
to allow building them as required.
Change-Id: I8f9768c09279f934f44b6c5b0ec363f7943bb796