arm: Avoid using the 'setend' instruction on ARMv7 and newer

This instruction is deprecated on ARMv8, and it is serializing on
some ARMv7 cores as well [1].

[1] http://article.gmane.org/gmane.linux.ports.arm.kernel/339293

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>