avutil: turn arm setend into a cpuflag

avutil: turn arm setend into a cpuflag

this allows disabling and enabling it
it also prevents crashes if vfpv3 and neon are disabled which previously
would have enabled the flag

And last but not least one can enable setend on cpus like cortex-a8 where
its fast but disabled by default

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>