ffmpeg: make ARM optimizations explicit

The ffmpeg configure script tries to autodetermine ARM optimizations by
building a small code snippet with AS.
If AS allows higher-level (>generic) assembly instructions then it
passes and enables the relevant optimization path.
Some toolchains allow/default to more than generic asm and then fail
when the real code is built.
Fixes:
http://autobuild.buildroot.net/results/b6e1225a07a58cc7544e37bc55675be71e2b5088/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>