arm: Don't clobber callee saved registers in scalarproduct

q4-q7/d8-d15 are supposed to not be clobbered by the callee.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit d307e408d4a9ada22df443cc38be77cc5e492694)